USNCO Part III Lab Practical Dashboard

Comprehensive analysis of all 18 lab practicals (2015-2025)

Total Labs

Quantitative

Qualitative

44%

Quant = Titration

56%

Qual = ID Unknowns

Lab Practicals

QUANT2015H₂O₂ Concentration — Catalytic Decomposition & Mass Lossmedium▶

Core Topic: Stoichiometry — Gravimetric Analysis via Catalytic Decomposition

Question: Determine the concentration (in mass %) of a hydrogen peroxide solution using catalytic decomposition.

What happens when MnO₂ is added to H₂O₂? Write the balanced equation.

Key Chemistry: 2H₂O₂ → 2H₂O + O₂ (catalyzed by MnO₂)

Gravimetric Decomposition Stoichiometry Mass Loss

Equipment

250 mL Erlenmeyer flasks (3)
Watch glasses (3) to cover flask mouths
Spatula, Beral pipets (2)
Electronic balance (±0.01 g, capacity ≥250 g)

Chemicals

100 mL hydrogen peroxide solution (6 mass %, labeled only "Hydrogen Peroxide Solution")
1 g MnO₂ catalyst in a vial

Given the equipment and chemicals, how would you design this experiment?

Method

Weigh flask + watch glass empty
Add measured H₂O₂ solution to flask; weigh again
Add small amount of MnO₂; cover with watch glass to prevent splashing
Wait for complete decomposition (no more bubbling)
Weigh final assembly — mass loss = mass of O₂ evolved
Calculate mass % H₂O₂ from stoichiometry
Replicate trials

What measurements would you record? Design your data table first.

Parameter	Trial 1	Trial 2	Trial 3
Mass flask + watch glass (g)	—	—	—
Mass flask + solution (g)	—	—	—
Mass H₂O₂ solution (g)	—	—	—
Mass after decomposition (g)	—	—	—
Mass O₂ lost (g)	—	—	—
Calculated mass % H₂O₂	—	—	—

Actual value: 6 mass %. From mass of O₂ lost, calculate moles O₂, then moles H₂O₂ (2:1 ratio), then mass H₂O₂ / mass solution × 100.

What would you expect to observe during this experiment?

Vigorous bubbling upon adding MnO₂ — exothermic reaction
Flask may become warm/hot — use watch glass to prevent loss
MnO₂ is a black powder that acts as catalyst (not consumed)
Decomposition can take several minutes to complete
Ensure all bubbling has stopped before final weighing
Small amounts of water vapor may also escape — source of error

What do you think the graders are looking for? How would you score this?

Point Breakdown (25 pts total)

Plan 5

Data 5

Calc 7

Answer 8

Expected answer: ~6 mass %. Students also asked to list sources of error and indicate directional effect on result.

Common Pitfalls

Not waiting for complete decomposition before final weighing
Forgetting to cover flask with watch glass — water vapor escapes, inflating mass loss
Using too much MnO₂ causes vigorous reaction and splashing/loss
Not doing replicate trials (minimum 2–3 for full credit)

QUAL2015Cu²⁺ Equilibrium — Le Chatelier's Principle & Unknown IDmedium▶

Core Topic: Chemical Equilibrium — Le Chatelier's Principle & Unknown Identification

Question: Determine the sign of ΔH for the [Cu(H₂O)₄]²⁺ ⇌ [CuCl₄]²⁻ equilibrium, and identify Solution A and Solution B as KCl or AgNO₃.

How does temperature shift the Cu²⁺ aqua/chloro equilibrium? What does that tell you about ΔH?

Key Chemistry: Blue [Cu(H₂O)₄]²⁺ ⇌ Green/Yellow [CuCl₄]²⁻ (color change indicates equilibrium shift)

Equilibrium Le Chatelier Color Observation Thermodynamics

Equipment

50 mL beakers (3)
Stirring rod, spatula
10 mL graduated cylinder
Beaker tongs
Access to hot plate (shared) and ice bath (shared)

Chemicals

NaCl (5 g solid)
0.3 M Cu(NO₃)₂ solution (50 mL)
Solution A: 0.1 M AgNO₃ (3 mL in Beral pipet)
Solution B: 2 M KCl (3 mL in Beral pipet)

Given the equipment and chemicals, how would you design this experiment?

Method

Prepare equilibrium mixture: dissolve NaCl in Cu(NO₃)₂ until color changes from blue to green
Split into 3 portions
Heat one portion, cool another — observe color changes to determine sign of ΔH
Add Solution A to one portion, Solution B to another — observe shifts
Identify A and B based on equilibrium shift direction

What measurements would you record? Design your data table first.

Condition	Color Change	Equilibrium Shift
Heating	More green/yellow	Forward (endothermic)
Cooling (ice bath)	More blue	Reverse (exothermic favored)
Add Solution A (AgNO₃)	More blue	Reverse (Ag⁺ removes Cl⁻)
Add Solution B (KCl)	More green	Forward (adds Cl⁻)

What would you expect to observe during this experiment?

Heating: Solution shifts toward green/yellow → forward reaction is endothermic → ΔH is positive
Cooling: Solution shifts toward blue → reverse reaction favored at low T
Solution A (AgNO₃): Ag⁺ precipitates Cl⁻ as AgCl → removes Cl⁻ → shifts left → more blue
Solution B (KCl): Adds Cl⁻ → shifts right → more green/yellow
Students should identify: [Cu(H₂O)₄]²⁺ = blue, [CuCl₄]²⁻ = green/yellow

What do you think the graders are looking for? How would you score this?

Point Breakdown (25 pts total)

Plan 5

Data/Obs 10

Answers 10

Must correctly identify species colors, sign of ΔH (positive), and both Solution A (AgNO₃) and Solution B (KCl).

Common Pitfalls

Confusing color direction: blue = [Cu(H₂O)₄]²⁺, green/yellow = [CuCl₄]²⁻
Not establishing a clear baseline color before adding reagents
Only testing heating OR cooling — must do both to confirm ΔH sign
Not connecting Ag⁺ removing Cl⁻ to the equilibrium shift

QUANT2016Fe³⁺/I⁻ Reaction Kinetics — Rate Order Determinationhard▶

Core Topic: Chemical Kinetics — Reaction Order Determination (Fe³⁺/I⁻)

Question: Determine the kinetic order with respect to [Fe³⁺] and [I⁻] using the Fe³⁺/I⁻/S₂O₃²⁻/starch system.

What is the rate law for 2Fe³⁺ + 2I⁻ → products? How does the starch-thiosulfate endpoint work?

Key Chemistry: 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂; thiosulfate scavenges I₂ until depleted, then starch turns blue-black

Kinetics Rate Law Method of Initial Rates Fe³⁺/I⁻ Clock

Equipment

10 mL graduated cylinders (2)
Disposable transfer pipettes (6)
50 or 100 mL beakers (2)
Stopwatch or timer

Chemicals

Solution A: 0.035 M Fe(NO₃)₃ in 0.3 M HNO₃ (100 mL)
Solution B: 0.060 M KI (100 mL)
Solution C: dilute Na₂S₂O₃ + starch indicator (100 mL)
Distilled water (500 mL)

Lab Setup

Given the equipment and chemicals, how would you design this experiment?

Method

Mix Solutions B and C in a beaker
Add Solution A and start timing
Record time until distinct, permanent blue-black color appears
Vary volumes of A (keeping B constant) to determine order w.r.t. [Fe³⁺]
Vary volumes of B (keeping A constant) to determine order w.r.t. [I⁻]
Use method of initial rates: compare rate ratios with concentration ratios

What measurements would you record? Design your data table first.

Trial	Vol A (mL)	Vol B (mL)	Vol C (mL)	Time (s)	Rate (1/t)
1	5	5	5	—	—
2	10	5	5	—	—
3	5	10	5	—	—

Students should keep total volume constant (use water to dilute) and calculate effective concentrations. Expected: first order in [Fe³⁺], second order in [I⁻].

What would you expect to observe during this experiment?

Solution changes from colorless/pale yellow to sudden blue-black endpoint
Thiosulfate scavenges I₂ as it forms — reaction appears to have no change until S₂O₃²⁻ is consumed
Starch-iodine complex produces the distinctive dark blue-black color
Doubling [Fe³⁺] should approximately halve the time (first order)
Doubling [I⁻] should approximately quarter the time (second order: 2² = 4× faster)
Temperature significantly affects rate — keep consistent

What do you think the graders are looking for? How would you score this?

Point Breakdown (25 pts total)

Plan 5

Data 7

Calc 7

Answer 6

Expected: Order w.r.t. [Fe³⁺] = 1, order w.r.t. [I⁻] = 2. Must also write rate expression: Rate = k[Fe³⁺][I⁻]²[S₂O₃²⁻]^x.

Common Pitfalls

Not keeping total volume constant across trials — must add water to compensate
Temperature variation between trials invalidates rate comparisons
Timing imprecision — the blue-black endpoint must be recorded to the second
Confusing the rate (1/t) calculation with the concentration change

QUAL2016Unknown Acid Classification — Mono/Di/Triproticmedium▶

Core Topic: Acid-Base Chemistry — Determining Proticity of an Unknown Acid

Question: Determine whether an unknown dilute acid is monoprotic, diprotic, or triprotic.

How many equivalence points would a monoprotic, diprotic, and triprotic acid each show?

Unknown: Phosphoric acid (H₃PO₄) — triprotic

Acid-Base Proticity Universal Indicator Titration

Equipment

10 mL graduated cylinders (2), transfer pipettes (6)
50 or 100 mL beakers (2)
Distilled water wash bottle

Chemicals

50 mL unknown dilute acid (phosphoric acid, ~0.5 N)
50 mL dilute NaOH solution (0.5 N)
Universal indicator solution (10 mL) with color chart

Given the equipment and chemicals, how would you design this experiment?

Method

Measure fixed volume of unknown acid
Add universal indicator
Titrate with NaOH dropwise, recording color changes
Count drops/volume to each equivalence point (color plateau change)
Number of distinct equivalence points = number of acidic protons
Alternative: prepare multiple aliquots with increasing ratios of NaOH to acid

What measurements would you record? Design your data table first.

Vol NaOH added (drops)	pH (est. from indicator)	Color
0	~2	Red
First equivalence	~4–5	Orange-Yellow
Second equivalence	~7	Yellow-Green
Third equivalence	~10	Blue-Violet

Three distinct color transitions indicate triprotic acid. Volume ratios between equivalence points should be approximately 1:1:1.

What would you expect to observe during this experiment?

Universal indicator changes through full color spectrum: red → orange → yellow → green → blue → violet
Three distinct color transition regions visible as NaOH is added
The third equivalence point may require significantly more NaOH (pK_a3 = 12.4 for H₃PO₄)
Students should look for pH "plateaus" followed by rapid changes
Color chart provided with indicator is essential for pH estimation

What do you think the graders are looking for? How would you score this?

Point Breakdown (25 pts total)

Plan 5

Data 8

Answer 4

Explain 8

Correct answer: triprotic. Must provide supporting data (volume ratios, color changes, graph/illustration) to justify conclusion.

Common Pitfalls

Being too sloppy with NaOH additions to detect the 3rd equivalence point
Not using small enough increments near color transitions
Not recording both volume AND color at each step
Forgetting that volume ratios between equivalence points should be ~1:1:1

QUANT2017Alka-Seltzer Composition — CO₂ Mass Loss Analysishard▶

Core Topic: Stoichiometry — Mass Loss (CO₂ Evolution) for Composition Analysis

Question: Determine the mass percent of sodium bicarbonate and citric acid in an Alka-Seltzer tablet.

Why do you need BOTH a water trial AND a vinegar trial? What does each tell you?

Key Chemistry: H₃A + 3NaHCO₃ → Na₃A + 3CO₂ + 3H₂O (citric acid is limiting in water alone)

Gravimetric CO₂ Evolution Stoichiometry Limiting Reagent

Equipment

Balance (±0.01 g)
50 mL graduated cylinder
250 or 400 mL beakers (2), watch glasses (2)
Spatula, Beral pipets (2), stirring rod

Chemicals

Alka-Seltzer tablets (6, original formula)
4.5–5% acetic acid (vinegar, 150 mL)
Sodium bicarbonate (3 g, for testing)

Lab Setup

Given the equipment and chemicals, how would you design this experiment?

Method

Weigh beaker + watch glass + tablet
Water trial: Dissolve tablet in water → citric acid is limiting → CO₂ mass loss = from citric acid only
Vinegar trial: Dissolve tablet in excess vinegar → all NaHCO₃ reacts → CO₂ mass loss = from all bicarbonate
Wait until no more bubbling; record final mass
Calculate mass % of NaHCO₃ and citric acid from stoichiometry

What measurements would you record? Design your data table first.

Exp	Water (mL)	Acid (mL)	Mass tablet (g)	Mass CO₂ (g)	Mass NaHCO₃ reacted (g)	Mass %
1 (water)	35	0	3.227	0.603	1.151	35.7
6 (vinegar)	10	25	3.254	0.999	1.909	58.6

Actual composition: NaHCO₃ = 59.1%, citric acid = 30.9%, aspirin = 325 mg remainder. Water-only trial gives citric acid mass %.

What would you expect to observe during this experiment?

Vigorous fizzing/bubbling when tablet dissolves (CO₂ evolution)
Small amount of solid may remain (aspirin is insoluble in water)
With water alone: fizzing stops when citric acid is consumed (limiting reagent)
With excess vinegar: more vigorous and prolonged fizzing — all NaHCO₃ reacts
Must wait until all visible gas evolution ceases before final weighing
Stirring helps release dissolved CO₂

What do you think the graders are looking for? How would you score this?

Point Breakdown (25 pts total)

Plan 4

Data 10

Calc 9

Ans 2

Expected Answers

Component	Actual Mass %
Sodium bicarbonate	59.1%
Citric acid	30.9%

Common Pitfalls

Only doing one solvent — need water (citric acid limiting) AND vinegar (all NaHCO₃ reacts)
Not stirring to release dissolved CO₂ before final weighing
Forgetting the limiting reagent concept: in water, citric acid limits; in excess acid, NaHCO₃ limits
Aspirin residue (insoluble) confusing mass measurements

QUAL2017Solution Identification — 5 Unknown 0.5 M Solutionsmedium▶

Core Topic: Qualitative Analysis — Solution Identification via Systematic Mixing

Question: Identify 5 unknown 0.5 M solutions (A–E) by mixing them pairwise and observing reactions.

Which pairs of CaCl₂, Na₂CO₃, NaCl, NaOH, H₂SO₄ produce precipitates or gas?

Unknowns: CaCl₂, Na₂CO₃, NaCl, NaOH, H₂SO₄

Qualitative Analysis Precipitation Gas Evolution Solubility Rules Net Ionic Equations

Equipment

Beral-style pipets (5)
24-well plate
Stirring rod

Chemicals

5 vials (A–E), each 5 mL of 0.5 M solutions
CaCl₂, Na₂CO₃, NaCl, NaOH, H₂SO₄

Given the equipment and chemicals, how would you design this experiment?

Method

Mix each pair systematically in well plate
Record all observations: precipitate, gas, heat, no change
Use a 5×5 grid matrix (10 unique combinations)
Identify based on reaction patterns

What measurements would you record? Design your data table first.

	CaCl₂	Na₂CO₃	H₂SO₄	NaOH
NaCl	No change	No change	No change	No change
CaCl₂	—	White ppt (thick)	No change	White ppt (fine/cloudy)
Na₂CO₃		—	Gas evolved (fizzes)	No change
H₂SO₄			—	No visible change (heat)

What would you expect to observe during this experiment?

Key Distinguishing Observations

NaCl: No reaction with any other solution — the "inert" one
Na₂CO₃ + H₂SO₄: Gas evolution (CO₂ fizzing) — unique identifier
CaCl₂ + Na₂CO₃: Thick white precipitate (CaCO₃)
CaCl₂ + NaOH: Fine white precipitate/cloudy (Ca(OH)₂)
H₂SO₄ + NaOH: Neutralization — heat but no visible change without indicator
Key ionic equations: Ca²⁺ + CO₃²⁻ → CaCO₃(s); CO₃²⁻ + 2H⁺ → CO₂(g) + H₂O

What do you think the graders are looking for? How would you score this?

Point Breakdown (25 pts total)

Plan 4

Data 7

ID 5

Equations 9

1 pt each for correct identification (5 pts). 9 pts for appropriate net ionic equations used in identification.

Common Pitfalls

Not testing ALL 10 unique pairs systematically
Cross-contaminating solutions with pipets
Not distinguishing "no visible reaction" from "both solutions are clear" in the grid
Forgetting to write net ionic equations for observed reactions

QUANT2018EDTA Titration — Ca²⁺/Mg²⁺ in Milkhard▶

Core Topic: Complexometric Titration — EDTA Determination of Divalent Cations

Question: Determine the total combined concentration (mol/L) of calcium and magnesium in skim milk.

How does EDTA bind metal ions? What role does Eriochrome Black T play at pH 10?

Key Chemistry: EDTA forms 1:1 complexes with Ca²⁺ and Mg²⁺ at pH 10; Eriochrome Black T indicator: red (metal-bound) → blue/black (free)

Titration EDTA Complexometry Indicator Real-World Sample

Equipment

10 mL graduated cylinder
25 or 50 mL Erlenmeyer flasks/beakers (3)
Stirring rod, well plate (12 or 24 wells)
Graduated Beral pipets (6, with 0.25 mL graduations)

Chemicals

0.0500 M EDTA solution (10 mL)
Ammonia buffer (pH ~10, 10 mL)
0.025 M MgCl₂ solution (10 mL, for practice)
Eriochrome Black T indicator (1 mL in Beral pipet)
Skim milk (0% fat, 5 mL)

Lab Setup

2018 USNCO Part III Problem 1 setup: EDTA solution, ammonia buffer, MgCl₂, cow's milk, Eriochrome Black T indicator, graduated cylinder, Erlenmeyer flasks, well plate

Given the equipment and chemicals, how would you design this experiment?

Method

Practice: titrate MgCl₂ + buffer + indicator with EDTA to learn endpoint
Mix measured volume of milk + buffer + indicator (turns red/wine color)
Titrate with EDTA drop by drop until color changes from red to blue/black
Record volume of EDTA; calculate moles of divalent cations
Alternative: add excess EDTA to milk, back-titrate with MgCl₂
Replicate trials

What measurements would you record? Design your data table first.

Parameter	Trial 1	Trial 2	Average
Volume milk (mL)	—	—	—
Volume buffer (mL)	—	—	—
Drops indicator	—	—	—
Volume EDTA to endpoint (mL)	—	—	—
Color at endpoint	Red → Blue	Red → Blue	—
Total [Ca²⁺+Mg²⁺] (mol/L)	—	—	—

USDA values: Ca 122–143 mg/100g, Mg 11–16 mg/100g in skim milk. Expected total: 0.036–0.044 mol/L.

What would you expect to observe during this experiment?

Milk + buffer + indicator → wine red / pink-red color
As EDTA is added: red color gradually fades
Endpoint: color changes from red/pink to blue/black (all metal ions complexed)
Milk opacity makes endpoint harder to read — use strong light
MgCl₂ practice run helps students learn to recognize the endpoint
Drop calibration essential — count drops and convert to volume

What do you think the graders are looking for? How would you score this?

Point Breakdown (25 pts total)

Plan 7

Data 6

Calc 6

Accuracy 6

Accuracy Tiers

Range	Points
0.030 – 0.045 M	Full credit (5/5)
0.020 – 0.030 or 0.045 – 0.060 M	3/5
0.015 – 0.020 or 0.060 – 0.070 M	2/5

Common Pitfalls

Not buffering to pH 10 — the indicator won't work at lower pH
Overshooting the endpoint: red → blue transition can be subtle
Not calibrating Beral pipet drop volume before starting
Forgetting that EDTA chelates BOTH Ca²⁺ and Mg²⁺ (total hardness)

QUAL2018Kool-Aid Chromatography — Dye Separation & Polarity Rankingeasy▶

Core Topic: Separation Science — Paper Chromatography & Polarity

Question: Separate dyes in two Kool-Aid samples using paper chromatography and rank the dyes by polarity.

Which FD&C dyes are in common Kool-Aid flavors? How does Rᶠ relate to polarity?

Dyes: Red 40, Yellow 5, Blue 1 (varies by flavor: Green Apple = Yellow 5 + Blue 1; Grape = Red 40 + Blue 1; Orange = Red 40 + Yellow 5)

Chromatography Polarity R_f Values Separation

Equipment

Whatman 3MM chromatography paper strips (12, 2×10 cm)
250 mL beakers (3) with watch glass covers
Cotton swabs (4), toothpicks (4)
Ruler, pencil, tweezers
25 or 50 mL graduated cylinder

Chemicals

Two Kool-Aid samples dissolved in water (0.4 g/mL, 1 mL each)
90% isopropyl alcohol (50 mL)
Distilled water

Lab Setup

2018 USNCO Part III Problem 2 setup: Kool-Aid samples (green apple, orange, grape), isopropyl alcohol, chromatography paper strips, cotton swabs, ruler, pencil, beakers

Given the equipment and chemicals, how would you design this experiment?

Method

Draw pencil line ~1 cm from bottom of paper strip
Spot Kool-Aid solution on line using cotton swab; let dry
Place strip in beaker with eluent (water, isopropanol, or mixtures) below the spot
Cover with watch glass; allow solvent to rise
Remove when solvent front near top; mark solvent front
Measure R_f values for each dye spot
Try multiple eluents for best separation

What measurements would you record? Design your data table first.

Dye	Color	R_f (water)	R_f (isopropanol)	Polarity Rank
Blue 1	Blue	Highest	Lowest	Least polar
Yellow 5	Yellow	Medium	Medium	Medium
Red 40	Red	Lowest	Medium	Most polar (water eluent)

Polarity order depends on eluent. Water eluent: Blue (least polar) < Yellow < Red (most polar). Isopropanol eluent: Blue < Red < Yellow.

What would you expect to observe during this experiment?

Each Kool-Aid flavor produces 2 distinct color spots on chromatography paper
With water eluent: more polar dyes travel farther (higher R_f)
With isopropanol: less polar dyes travel farther
Common shared dye between flavors visible at same R_f position
Best separation often with mixed eluent (e.g., 70:30 isopropanol:water)
Over-concentrated spots give tailing; let spots dry between applications

What do you think the graders are looking for? How would you score this?

Point Breakdown (25 pts total)

Plan 7

Data/Obs 11

Dyes 3

Rank 4

Must identify number and color of dyes in each sample (3 pts), whether shared dyes exist (2 pts), and rank dyes by polarity (2 pts).

Common Pitfalls

Making spots too large or too wet on the paper
Paper touching beaker walls during development
Not running chromatography long enough for full separation
Using pen (not pencil) for the origin line — ink will run with solvent

QUANT2019Calorimetry — Enthalpy of H₂O₂ Decompositionmedium▶

Core Topic: Thermochemistry — Calorimetry & Enthalpy of Decomposition

Question: Determine the enthalpy of decomposition of hydrogen peroxide using coffee cup calorimetry.

Write the H₂O₂ decomposition equation. Is it exo- or endothermic?

Key Chemistry: H₂O₂(aq) → H₂O(l) + ½O₂(g), catalyzed by Fe(NO₃)₃

Thermochemistry Calorimetry q = mcΔT Catalyst

Equipment

Styrofoam cups (4) + cardboard lids with hole
Thermometer
10 mL and 50 mL graduated cylinders
Magnetic stirrer/hot plate, stir bar
150 or 250 mL beakers (2)
Balance (±0.01 g)

Chemicals

3% H₂O₂ (150 mL)
0.5 M Fe(NO₃)₃ (30 mL)

Lab Setup

2019 USNCO Part III Problem 1 setup: hot plate/stirrer, thermometer, Erlenmeyer flasks, timer for H₂O₂ decomposition calorimetry

Given the equipment and chemicals, how would you design this experiment?

Method

Calibrate calorimeter: Mix known amounts of warm and room-temp water; measure ΔT to determine C_cal
Measure 50 mL H₂O₂; record T_initial
Add 10 mL of 0.5 M Fe(NO₃)₃; cap and stir
Record T_final (maximum temperature)
q_total = q_solution + q_calorimeter
Calculate moles H₂O₂ from 3% solution; ΔH = −q_total/n

What measurements would you record? Design your data table first.

Parameter	Calibration	Decomposition
Cold/RT water (mL)	50.0	50.0 (H₂O₂)
T_initial (°C)	22.4 / 39.6	20.0
Hot water / Fe(NO₃)₃ (mL)	50.0	10.0
T_final (°C)	30.5	34.8
C_cal (J/°C)	25.8	—
q_total (J)	—	4097
Moles H₂O₂	—	0.04407
ΔH (kJ/mol)	—	−93.0

Literature value: −94.6 kJ/mol. Molarity of 3% H₂O₂ ≈ 0.8813 M.

What would you expect to observe during this experiment?

Addition of Fe(NO₃)₃ causes vigorous bubbling (O₂ evolution)
Solution color: pale yellow → dark amber → back to pale yellow (catalyst cycle)
Temperature rises significantly (exothermic decomposition)
Color change back to pale yellow confirms Fe³⁺ is a catalyst (regenerated)
Higher Fe(NO₃)₃ concentration → faster reaction but same ΔH
~0.08 M Fe³⁺ gives excellent results

What do you think the graders are looking for? How would you score this?

Point Breakdown (25 pts total)

Plan 7

Data 6

Calc 7

Answer 5

Expected Answer

Value	Literature	Acceptable Range
ΔH decomposition	−94.6 kJ/mol	−92.7 to −96.5 kJ/mol

Must also explain role of Fe(NO₃)₃ as catalyst with experimental evidence (color change cycle, rate increase).

Common Pitfalls

Not accounting for the calorimeter heat capacity
Adding catalyst too fast — reaction may be violent and cause splashing
Not stirring during reaction — uneven temperature reading
Temperature reading lag if using alcohol thermometer

QUAL2019Polymer Absorption — Sodium Polyacrylate Swellingeasy▶

Core Topic: Polymer Chemistry — Superabsorbent Polymer & Ionic Effects on Swelling

Question: Rank the ability of different aqueous solutions to be absorbed by sodium polyacrylate polymer.

How does ionic strength affect polymer swelling? What role does charge play?

Solutions: 0.05 M NaCl, 0.05 M NH₄Cl, 0.05 M MgCl₂, 0.05 M glucose, distilled water

Polymer Osmotic Pressure Ionic Strength Cross-Linking

Equipment

Watch glasses (4), test tubes (4)
Beral pipets (4), spatula, stirring rod
Balance (±0.01 g)

Chemicals

Sodium polyacrylate powder (1 g)
0.05 M NaCl (25 mL)
0.05 M NH₄Cl (25 mL)
0.05 M MgCl₂ (25 mL)
0.05 M glucose (25 mL)
Distilled water

Lab Setup

2019 USNCO Part III Problem 2 setup: test tubes in wooden rack with unknown solutions, watch glass, pipets for polymer absorption experiment

Given the equipment and chemicals, how would you design this experiment?

Method

Place equal amounts of polymer (~spatula full or weighed) on separate watch glasses
Add each solution dropwise until polymer no longer absorbs
Count drops or measure volume absorbed for each solution
Alternative: add known volume of solution, then add polymer until solidified
Replicate trials for reliability

What measurements would you record? Design your data table first.

Solution	Drops Absorbed	Mass Absorbed per g Polymer	Rank
Distilled water	26–27	~410 g/g	Most absorbed
Glucose (0.05 M)	30–32	>310 g/g	2nd
NaCl (0.05 M)	8	~110 g/g	3rd
NH₄Cl (0.05 M)	6–7	~110 g/g	4th
MgCl₂ (0.05 M)	3–4	~50 g/g	Least absorbed

What would you expect to observe during this experiment?

Water: Maximum absorption — polymer swells dramatically into gel
Glucose: Nearly as much absorption as water (non-ionic solute)
NaCl / NH₄Cl: Significantly less absorption — monovalent cations reduce osmotic swelling
MgCl₂: Least absorption — divalent Mg²⁺ cross-links carboxylate groups, collapsing polymer network
Trend: Ionic solutions reduce swelling; higher charge density → more cross-linking → less absorption
Polymer gel should not go down drain (clogs pipes)

What do you think the graders are looking for? How would you score this?

Point Breakdown (25 pts total)

Plan 7

Data 7

Rank 5

Explain 6

Correct ranking: Mg²⁺ < NH₄⁺ ≤ Na⁺ < glucose ≤ water. Must explain trend using osmotic pressure and cross-linking principles.

Common Pitfalls

Not using equal masses of polymer for each solution test
Not allowing enough time for full swelling equilibrium
Confusing ionic strength (charge matters) with molarity (0.05 M for all)
Not explaining WHY divalent ions reduce swelling more than monovalent

QUANT2022Antacid Back-Titration — Moles of HCl Neutralizedhard▶

Core Topic: Acid-Base Chemistry — Back Titration

Question: Determine the moles of HCl (simulating stomach acid) that can be neutralized by a tablet of commercial antacid (TUMS).

Write CaCO₃ + 2HCl reaction. Why is back-titration needed instead of direct?

Key Chemistry: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

Acid-Base Back Titration Stoichiometry No Balance Provided

Equipment

25 mL graduated cylinder
Graduated Beral pipets (0.25 mL graduations)
100–150 mL beakers (3)
Hot plate (no stirring)
pH indicator strips (range 1–12)
Glass stirring rod, spatula, wax paper

Chemicals

Standardized 0.5 M HCl (100 mL)
Standardized 0.40–0.45 M NaOH (50 mL)
Phenolphthalein indicator (0.5 mL)
TUMS peppermint tablets (3, removed from packaging)

Lab Setup

2022 USNCO Part III Problem 1 setup: pH indicator strips, antacid tablets, beakers, pipets, balance on green tray

2022 USNCO Part III shared equipment: paper towels, phenolphthalein indicator, distilled water, stirring rod

Given the equipment and chemicals, how would you design this experiment?

Method (Back Titration)

Crush tablet; dissolve in measured excess of standardized HCl
Heat to near boiling to expel CO₂; cool
Check with pH paper that excess acid remains
Add phenolphthalein; titrate with standardized NaOH to pink endpoint
Moles HCl reacted = Total moles HCl − moles excess HCl (= moles NaOH at endpoint)
Replicate trials

Alternative method: Mass loss — weigh before/after reaction to determine CO₂ lost, then calculate moles CaCO₃ and hence moles HCl.

What measurements would you record? Design your data table first.

Students should record:

Parameter	Trial 1	Trial 2
Volume HCl used (mL)	—	—
Volume NaOH to endpoint (mL)	—	—
Drops → volume conversion	—	—
Color change observation	—	—

Note: No balance was provided — students must work with volumes and concentrations only. Mass of tablet given as 1.3 g.

What would you expect to observe during this experiment?

Tablet fizzes when added to HCl (CO₂ evolution)
Solution may appear white/cloudy due to tablet fillers
Heating drives off dissolved CO₂ — watch for bumping
Phenolphthalein endpoint: colorless → persistent pale pink
Fillers may burn if overheated on hot plate
pH paper should confirm excess acid before titration

What do you think the graders are looking for? How would you score this?

Point Breakdown (25 pts total)

Plan 5

Data 6

Calc 7

Accuracy 7

Accuracy Tiers (moles HCl per 1.3 g tablet)

Range	Points
0.00980 – 0.0106 mol	7/7 (full)
0.00882 – 0.00979 or 0.0107 – 0.0116 mol	5/7
0.00784 – 0.00881 or 0.0117 – 0.0127 mol	3/7
Any other value	0/7

Actual value: ~0.0100 mol. Answers in other units converted to moles before grading.

Common Pitfalls

Not dissolving TUMS completely in HCl before back-titrating
Heating too aggressively — bumping causes solution loss
No balance provided — must rely on volumetric measurements only
Not using enough excess HCl (all CaCO₃ must react first)

QUAL2022White Powder Identification — 6 Unknownsmedium▶

Core Topic: Qualitative Analysis — Solid Identification via Chemical Reagent Testing

Question: Identify 6 unknown white powders using provided reagents.

Which reagent tests distinguish NaHCO₃, borax, cornstarch, epsom salts, sugar, and washing soda?

Unknowns: Baking soda (NaHCO₃), Borax (Na₂B₄O₇), Cornstarch, Epsom salts (MgSO₄), Powdered sugar (sucrose + cornstarch), Washing soda (Na₂CO₃)

Qualitative Analysis Solubility Acid-Base Indicators Iodine-Starch Test Precipitation

Reagents Provided

Distilled water
10% HCl
10% NaOH
2% Iodine-KI solution
Bromothymol blue indicator
Phenolphthalein indicator

Equipment

12 test tubes + rack
Well plate (6 or 12 wells)
4 Beral pipets
Glass stirring rod, spatula

Lab Setup

2022 USNCO Part III Problem 2 setup: unknown solution bottles, bromothymol blue indicator, test tubes in rack, well plate, Beral pipets on green tray

Given the equipment and chemicals, how would you design this experiment?

Method

Use equal small amounts of each unknown
Test each with every reagent, add dropwise
Observe: solubility, gas production, precipitate, color change
Record systematically in a grid/flow chart

What measurements would you record? Design your data table first.

Test \ Unknown	Cornstarch	NaHCO₃	MgSO₄	Pwdr Sugar	Na₂CO₃	Borax
Water	Insoluble	Soluble	Soluble	Soluble	Soluble	Sparingly sol.
10% HCl	Insoluble	Sol. + gas	Soluble	Soluble	Sol. + gas	Sol., no gas
10% NaOH	No change	No change	White ppt	No change	No change	No change
Iodine	Dark blue-black	No change	No change	Faint blue-black	No change	No change
Phenolphthalein	No change	Pale pink	No change	No change	Dark pink	Pink
Bromothymol blue	Yellow-green	Blue	Yellow	Blue	Blue	Blue

What would you expect to observe during this experiment?

Key Distinguishing Observations

Cornstarch: Only insoluble powder + dark blue-black with iodine
Powdered sugar: Soluble + faint blue-black with iodine (contains some cornstarch)
NaHCO₃ vs Na₂CO₃: Both gas with HCl, but Na₂CO₃ → dark pink with phenolphthalein, NaHCO₃ → pale pink
Epsom salts (MgSO₄): Only powder forming white precipitate with NaOH: Mg(OH)₂
Borax: Soluble in acid but no gas; pink with phenolphthalein

What do you think the graders are looking for? How would you score this?

Point Breakdown (25 pts total)

Plan 5

Data 8

ID + Justify 12

Each correct identification with valid justification: 2 pts per unknown (6 unknowns = 12 pts).

Common Pitfalls

Missing the iodine-starch test (turns purple/black) for cornstarch and powdered sugar
Not testing pH of dissolved solutions — easy way to distinguish carbonates
Cross-contaminating samples on well plate
Not testing with HCl for fizzing (distinguishes carbonates from non-carbonates)

QUANT2023Weak Acid Titration — Molar Mass & pK_amedium▶

Core Topic: Acid-Base Titration — Molar Mass Determination & pK_a

Question: Determine the molar mass and pK_a of an unknown monoprotic weak acid.

How do you find both molar mass AND pKₐ from a single titration?

Unknown: Benzoic acid (C₆H₅COOH, MM = 122.12 g/mol, pK_a = 4.2)

Acid-Base Direct Titration Molar Mass Half-Equivalence Point Indicators

Equipment

10 mL graduated cylinder, Beral pipets (6)
Balance (nearest 0.01 g)
50 or 100 mL beakers (6), stirring rods (3)
Well plate, ruler, spatula, weigh paper

Chemicals

Solid benzoic acid (labeled "unknown monoprotic acid"), 0.5 g
Standardized 1 M NaOH (20 mL)
Phenolphthalein indicator
Methyl orange indicator

Lab Setup

2023 USNCO Part III Problem 1 materials: standardized NaOH, methyl orange indicator, unknown monoprotic acid, spatula

2023 USNCO Part III shared materials: DI water, beakers, Beral pipets, well plate, paper towels

Given the equipment and chemicals, how would you design this experiment?

Method

Determine volume per drop of Beral pipet
Weigh ~0.10 g of unknown acid; dissolve/suspend in water
Add phenolphthalein; titrate with 1 M NaOH counting drops to pink endpoint
Calculate: moles NaOH = moles acid (monoprotic) → molar mass = mass/moles
For pK_a: repeat with fresh acid sample, add exactly half the drops → pH at half-equivalence = pK_a
Use methyl orange color to estimate pH ~4

What measurements would you record? Design your data table first.

Parameter	Trial 1	Trial 2	Trial 3	Average
Drops NaOH to endpoint	18	15	16	16.3
Volume NaOH (L)	0.00090	0.00075	0.00080	0.00082
Moles NaOH	0.00095	0.00080	0.00085	0.00087
Mass acid (g)	0.10	0.10	0.10	0.10
Molar mass (g/mol)	105	126	118	116

At half-equivalence (~8 drops): phenolphthalein colorless, methyl orange yellowish-orange → pK_a ≈ 4

What would you expect to observe during this experiment?

Benzoic acid is sparingly soluble in water; may see suspended particles
Phenolphthalein endpoint: colorless → pink
At half-equivalence: methyl orange shows yellowish-orange (transition range 3.1–4.4)
Drop counting precision is critical — large drops cause overshoot
Students may attempt to use both indicators simultaneously or sequentially

What do you think the graders are looking for? How would you score this?

Expected Answers

Value	Actual	Acceptable Range
Molar Mass	122.12 g/mol	~105–130 g/mol (sample data)
pK_a	4.2	~3.5–4.5 (indicator-based estimate)

Rubric emphasizes: drop calibration, replicate trials, clear calculations, correct identification of half-equivalence point concept.

Common Pitfalls

Not finding the half-equivalence point (where pH = pKₐ)
Incorrect molar mass calculation — must use moles NaOH at equivalence = moles acid
Not weighing the acid sample accurately enough
Choosing the wrong indicator for the specific pKₐ range

QUAL2023Sodium Alginate Spherification — Cross-Linking Strengtheasy▶

Core Topic: Polymer Chemistry — Ionic Cross-Linking & Molecular Gastronomy

Question: Determine which cation (Ca²⁺, Mg²⁺, K⁺) produces the strongest cross-links in sodium alginate polymer to form hydrogel spheres.

Polymer Chemistry Ionic Cross-Linking Charge Density Semi-Quantitative

Chemicals

2% sodium alginate solution (10 mL, colored with food dye)
2 M CaCl₂ (30 mL)
2 M MgCl₂ (30 mL)
2 M KCl (30 mL)

Lab Setup

2023 USNCO Part III Problem 2 materials: sodium alginate solution (green), CaCl₂, MgCl₂, KCl bottles, watch glass, spoon, ruler

Given the equipment and chemicals, how would you design this experiment?

Method

Drop sodium alginate solution into each cation solution
Compare sphere quality: firmness, shape retention, elasticity
Best approach: create dilution series to find minimum concentration for spherification
Can also dilute alginate and test minimum concentration

What measurements would you record? Design your data table first.

Cation	Sphere Formation	Minimum Conc. for Spheres
Ca²⁺	Well-formed, elastic spheres	~0.04–0.1 M
Mg²⁺	Sheets/films only, no spheres	N/A
K⁺	No spheres at any concentration	N/A

What would you expect to observe during this experiment?

Ca²⁺: Strong, elastic spheres at 0.1 M+; softer at 0.04–0.09 M; none below 0.04 M
Mg²⁺: No discrete spheres — forms sheets or films instead
K⁺: No cross-linking observed at any concentration
Explanation: Ca²⁺ (divalent, appropriate ionic radius) bridges carboxylate groups on adjacent alginate chains effectively. Mg²⁺ is divalent but too small for effective "egg-box" coordination. K⁺ is monovalent and cannot bridge chains.

What do you think the graders are looking for? How would you score this?

Assessment Areas

Plan: Systematic approach, dilution series concept
Data: Clear observations for each cation, concentration effects
Answer: Ca²⁺ produces strongest cross-links
Explanation: Relates ionic charge and size to cross-linking ability; mentions "egg-box" model or carboxylate bridging

Common Pitfalls

Only testing undiluted cation solutions — dilution series gives much better data
Not quantifying sphere quality consistently across cations
Not explaining the "egg-box" model or why charge density matters
Forgetting to note that Mg²⁺ forms sheets/films, not spheres

QUANT2024Calorimetry — Enthalpy of Solution for 3 Unknownseasy▶

Core Topic: Thermochemistry — Calorimetry & Enthalpy of Solution

Question: Determine the enthalpy of (heat of) solution, in kJ/mol, for three unknown solids.

Which of urea, Na₂CO₃, and NaOAc dissolutions are endothermic vs exothermic?

Unknowns: Urea (+13.9 kJ/mol), Na₂CO₃ (−26.7 kJ/mol), NaOAc (−17.3 kJ/mol)

Thermochemistry Calorimetry q = mcΔT Endo/Exothermic

Equipment

Thermometer (alcohol or digital)
Graduated cylinder (25 mL+)
Styrofoam cup calorimeter with cap
250 mL+ beaker, stirring rod
Balance (milligram precision)

Given the equipment and chemicals, how would you design this experiment?

Method

Measure and weigh water (~25 mL) in calorimeter cup
Record initial temperature
Weigh ~2 g of unknown solid
Add solid to water; stir; cap
Record maximum/minimum temperature
Calculate: q_water = mcΔT; q_dissolution = −q_water
Convert to kJ/mol using mass and molar mass
Replicate trials for each unknown

What measurements would you record? Design your data table first.

Parameter	Unk 1 T1	Unk 1 T2	Unk 2 T1	Unk 2 T2	Unk 3 T1	Unk 3 T2
Mass water (g)	25.15	25.85	27.59	26.42	22.67	24.62
T_i (°C)	22.4	22.3	22.5	22.4	22.2	22.3
Mass solid (g)	2.06	2.15	2.18	2.20	2.26	2.05
T_f (°C)	17.3	17.2	27.6	27.8	27.0	26.9

What would you expect to observe during this experiment?

Unknown 1 (Urea): Temperature decreases — endothermic dissolution
Unknown 2 (Na₂CO₃): Temperature increases — exothermic dissolution
Unknown 3 (NaOAc): Temperature increases — exothermic dissolution
Cup should be capped to minimize heat loss
Stirring ensures uniform dissolution and temperature reading

What do you think the graders are looking for? How would you score this?

Expected Answers

Unknown	Identity	ΔH_sol (kJ/mol)	Sample Calc (kJ/mol)
1	Urea	+13.9	+15.6
2	Na₂CO₃	−26.7	−28.6
3	NaOAc	−17.3	−16.5

Rubric emphasizes: proper calorimetry setup, correct sign convention (endo vs exo), replicate measurements, clear q = mcΔT calculations.

Common Pitfalls

Not capping the Styrofoam cup calorimeter — heat escapes
Wrong sign convention: q_dissolution = −q_water
Not using enough water to fully dissolve the solid
Forgetting replicate trials for each unknown

QUAL2024Capillary Tube Liquid Identification — Surface Tensionmedium▶

Core Topic: Intermolecular Forces — Surface Tension & Capillary Action

Question: Identify 5 unknown liquids using capillary tube behavior.

Rank water, salt water, vegetable oil, isopropanol, and acetone by surface tension.

Unknowns: Water, 10% salt water, vegetable oil, isopropanol, acetone

IMF Surface Tension Capillary Action Physical Properties

Equipment

5 open-ended glass capillary tubes
Ruler with mm markings
5 unknown liquid samples (~1 mL each)

Given the equipment and chemicals, how would you design this experiment?

Method

Dip capillary tube vertically into each liquid
Measure the height of capillary rise (mm)
Higher surface tension + lower density → greater capillary rise
Compare results to known surface tension values
Also observe: meniscus shape, viscosity, evaporation rate

What measurements would you record? Design your data table first.

Liquid	Surface Tension (mN/m)	Expected Rise Order	Other Clues
Water	72.8	Highest	Concave meniscus
10% Salt water	~74	Very high (close to water)	Slightly higher tension than pure water
Isopropanol	23.0	Moderate	Alcohol smell, fast evaporation
Acetone	25.2	Moderate	Strong smell, very fast evaporation
Vegetable oil	~32	Low (high viscosity offsets)	Viscous, slow rise, no evaporation

Note: Capillary rise depends on surface tension, density, and contact angle. Oil has moderate surface tension but high viscosity slows the rise.

What would you expect to observe during this experiment?

Water vs salt water: Very similar rise heights — distinguish by slight density/tension difference or taste is not allowed; students may struggle here
Isopropanol vs acetone: Similar surface tensions; acetone evaporates faster and has distinct smell
Vegetable oil: Visibly viscous, slow rise, no evaporation, distinctive appearance
Additional observations: meniscus curvature (concave for hydrophilic liquids), evaporation speed, viscosity

What do you think the graders are looking for? How would you score this?

Assessment Areas

Plan: Systematic capillary rise measurement, controlled procedure
Data: Measured heights (mm) for all unknowns, multiple trials
Calculations: Ranking by surface tension, relating to IMF
Identification: Correct ID with justification from capillary data + supplementary observations

Common Pitfalls

Not holding capillary tube perfectly vertical
Reading height from wrong point on the meniscus
Not using supplementary observations: smell, viscosity, evaporation rate
Difficulty distinguishing water from salt water — very similar capillary rise

QUANT2025Buffer Analysis — Capacity, [HA], [A^-], K_ahard▶

Core Topic: Acid-Base Equilibrium — Buffer Capacity, Henderson-Hasselbalch

Question: Determine the buffer capacity, concentrations of conjugate acid and base, and K_a of the conjugate acid for an unknown buffer solution.

Write the Henderson-Hasselbalch equation. How do you find Kₐ from a buffer titration?

Buffer: pH ≈ 9, K_a ≈ 6 × 10⁻¹⁰

Acid-Base Equilibrium Buffer Henderson-Hasselbalch Dropwise Titration

Equipment

Graduated Beral pipets
pH paper (range 0–14)
Beakers, stirring rods, wash bottle
Graduated cylinder (10 mL)

Chemicals

Unknown buffer solution
1.0 M HCl
1.0 M NaOH

Given the equipment and chemicals, how would you design this experiment?

Method

Calibrate drop volume (drops per mL)
Measure 10 mL of buffer; record initial pH
Titrate with 1.0 M NaOH, one drop at a time, recording pH after each drop
Repeat with fresh buffer + 1.0 M HCl
Plot pH vs drops; identify equivalence points and buffer region
Calculate buffer capacity, [HA], [A⁻], and K_a

What measurements would you record? Design your data table first.

Drops NaOH	pH
0	9
1–6	9 → 9
7	9.5
8	10
9	11
10–13	11.5 → 12

Drops HCl	pH
0	9
1–3	9 → 9
4	8
5	6.5
6	4
7–9	3 → 3

Equivalence: ~8.5 drops NaOH (neutralizes HA), ~5 drops HCl (neutralizes A⁻)

What would you expect to observe during this experiment?

Buffer resists pH change for first several drops of both acid and base
Sharp pH jump at equivalence point — dramatic color change on pH paper
NaOH titration: 6–7 drops in buffering region before jump
HCl titration: 4–5 drops in buffering region before jump
Buffer capacity asymmetric: more HA than A⁻

What do you think the graders are looking for? How would you score this?

Point Breakdown (30 pts total)

Plan 5

Data 5

Cap. 5

Conc. 5

Ka 5

Ans 5

Expected Answers

Quantity	Expected Value
Buffer capacity	~0.05 (buffer range pH 8–10)
[HA]	~0.040 M
[A⁻]	~0.025 M
K_a	~6 × 10⁻¹⁰

Common Pitfalls

Not calibrating drop volume (drops/mL) before starting titration
Not using fresh buffer aliquot for each titration direction (acid and base)
Missing that pH at half-equivalence = pKₐ
Confusing which equivalence point corresponds to [HA] vs [A⁻]

QUAL2025Salt Solution Identification — 4 Unknown Saltsmedium▶

Core Topic: Inorganic Qualitative Analysis — Salt Identification via Reagent Testing

Question: Identify 4 unknown salt solutions using provided reagents. Each cation and anion used only once.

What reagents distinguish Na⁺, Ca²⁺, Mg²⁺, and NH₄⁺? What about OAc⁻, CO₃²⁻, Cl⁻, NO₃⁻?

Unknowns: MgCl₂, Ca(OAc)₂, NH₄NO₃, Na₂CO₃

Ion pool: Na⁺, Ca²⁺, Mg²⁺, NH₄⁺ | OAc⁻, CO₃²⁻, Cl⁻, NO₃⁻

Qualitative Analysis Precipitation Acid-Base Solubility Rules Process of Elimination

Reagents Provided

0.01 M AgNO₃
1.0 M HCl
0.1 M NaOH
1.0 M NaOH
0.1 M Na₃PO₄
pH strips

Given the equipment and chemicals, how would you design this experiment?

Method

Test pH of each unknown
Test each unknown with each reagent in a spot plate
Record: precipitates, gas evolution, smell
Use process of elimination for cations and anions

What measurements would you record? Design your data table first.

Test \ Unknown	MgCl₂	Ca(OAc)₂	NH₄NO₃	Na₂CO₃
pH	6–7	6	4–5	11
AgNO₃	ppt	No rxn (faint)	No rxn	ppt
1.0 M HCl	No rxn	No rxn (vinegar smell)	No rxn	Bubbles!
0.1 M NaOH	ppt	No/faint ppt	No rxn	No rxn
1.0 M NaOH	ppt	ppt	NH₃ smell	No rxn
Na₃PO₄	ppt	ppt	No rxn	No rxn

What would you expect to observe during this experiment?

Key Distinguishing Observations

Na₂CO₃: pH 11 (basic anion) + bubbles with HCl → unique CO₃²⁻ identifier
NH₄NO₃: pH 4–5 (acidic cation) + ammonia smell with 1.0 M NaOH + no precipitates
MgCl₂: ppt with 0.1 M NaOH (Mg(OH)₂) + ppt with AgNO₃ (AgCl)
Ca(OAc)₂: No ppt with 0.1 M NaOH but ppt with 1.0 M NaOH; vinegar smell with HCl; AgOAc slightly soluble (faint/no ppt)
Distinguishing Mg²⁺ from Ca²⁺: 0.1 M NaOH precipitates Mg(OH)₂ but Ca(OH)₂ is "soluble" enough to stay dissolved at 0.1 M

What do you think the graders are looking for? How would you score this?

Point Breakdown (25 pts total)

Plan 4

Data 10

ID+Just 10

Must note reagent concentrations in data table (−1 if omitted). Each correct salt + valid justification: 2.5 pts (4 unknowns = 10 pts).

Common Pitfalls

Not testing pH first — easy way to narrow down (Na₂CO₃ is basic)
Missing the ammonia smell test: warm NH₄NO₃ + NaOH releases NH₃ gas
Not using process of elimination (each ion appears only once)
Forgetting AgNO₃ test for Cl⁻ (white AgCl precipitate)

Year-by-Year Comparison

Year	Type	Topic	Core Technique	Unknowns	Balance?	Key Skill
2015	QUANT	H₂O₂ concentration	Catalytic decomposition / mass loss	0	Yes	Gravimetric analysis, stoichiometry
2015	QUAL	Cu²⁺ equilibrium	Le Chatelier observation	2	No	Equilibrium, color observation, thermodynamics
2016	QUANT	Iodine clock kinetics	Method of initial rates	0	No	Kinetics, rate law, timing
2016	QUAL	Acid proticity	Titration + universal indicator	1	No	Acid-base, indicator color mapping
2017	QUANT	Alka-Seltzer composition	CO₂ mass loss	0	Yes	Stoichiometry, limiting reagent
2017	QUAL	Solution ID (5 unknowns)	Systematic mixing	5	No	Precipitation, gas evolution, solubility rules
2018	QUANT	Ca²⁺/Mg²⁺ in milk	EDTA titration	0	No	Complexometric titration, indicator
2018	QUAL	Kool-Aid dye separation	Paper chromatography	2 flavors	No	Chromatography, R_f, polarity
2019	QUANT	H₂O₂ ΔH decomposition	Calorimetry	0	Yes	q=mcΔT, catalyst, calorimeter constant
2019	QUAL	Polymer absorption ranking	Superabsorbent swelling	5 solutions	Yes	Polymer chemistry, osmotic pressure, ionic effects
2022	QUANT	Antacid neutralization	Back titration	0	No	Titration, stoichiometry
2022	QUAL	White powder ID	Reagent testing	6	No	Chemical observations, systematic testing
2023	QUANT	Weak acid analysis	Direct titration	1	Yes	Titration, molar mass, pKa
2023	QUAL	Polymer cross-linking	Spherification	3 cations	No	Observation, dilution series, reasoning
2024	QUANT	Enthalpy of solution	Calorimetry	3	Yes	q=mcΔT, sign convention
2024	QUAL	Liquid ID	Capillary action	5	No	IMF, surface tension, physical observation
2025	QUANT	Buffer analysis	Dropwise titration	0	No	Buffer chemistry, Henderson-Hasselbalch
2025	QUAL	Salt solution ID	Reagent testing	4	No	Precipitation, solubility rules, elimination

Trends & Statistics

Titration-based quantitative labs

2018 (EDTA), 2022 (back), 2023 (direct), 2025 (buffer)

4 / 9 quant labs (44%)

Mass loss / gravimetric analysis

2015 (H₂O₂ decomposition), 2017 (Alka-Seltzer CO₂)

2 / 9 quant labs (22%)

Calorimetry

2019 (H₂O₂ ΔH), 2024 (enthalpy of solution)

2 / 9 quant labs (22%)

Unknown/substance identification dominates qualitative

2015 (2 solutions), 2016 (acid type), 2017 (5 solutions), 2022 (6 powders), 2024 (5 liquids), 2025 (4 salts)

6 / 9 qual labs (67%)

Each year pairs one quant + one qual

Consistent format across all 9 years

9 / 9 years (100%)

Drop counting as volume measurement

Used in 2018, 2022, 2023, 2025 — students must calibrate drop volume

4 / 9 years (44%)

No buret ever provided

All titrations done with Beral pipets or graduated cylinders

9 / 9 years (100%)

Replicate trials expected

Every quantitative lab rubric rewards multiple trials

9 / 9 years (100%)

Balance provided in most years

Available in 2015, 2017, 2018, 2019, 2023, 2024; absent in 2016, 2022, 2025

6 / 9 years (67%)

Qualitative labs require systematic observation tables

Grid/matrix format strongly rewarded for identification problems

9 / 9 years (100%)

Real-world samples

Alka-Seltzer (2017), milk (2018), Kool-Aid (2018), TUMS (2022), diapers (2019)

5 / 9 years (56%)

Kinetics tested once

2016 (Fe³⁺/I⁻) — only year testing reaction rate/order

1 / 9 years (11%)

Quantitative Labs by Year

Qualitative Labs by Year

Skills Frequency Across All Labs

Average Rubric Weight Distribution

Loading charts...

Coaching Recommendations

Priority Practice #1: Titration Mastery

Practice with Beral pipets (not burets!) — calibrate drop volume every time
Back titration, direct titration, and buffer titration are all fair game
Know phenolphthalein, methyl orange, bromothymol blue endpoints cold
Practice reaching endpoints within 15–20 drops for accuracy
Always do at least 2 replicate trials

Priority Practice #2: Qualitative Analysis

Build a mental flowchart for common ions: Ag⁺, Cl⁻, CO₃²⁻, SO₄²⁻, NH₄⁺
Practice reagent-by-reagent grid testing on spot plates
Know solubility rules: which hydroxides, carbonates, sulfates, chlorides are insoluble
Smell tests: acetic acid (vinegar), ammonia — use wafting technique
Iodine-starch test, flame tests, precipitation reactions

Priority Practice #3: Calorimetry

Styrofoam cup calorimetry — proper technique for temperature measurement
q = mcΔT — know sign conventions (endo vs exo) cold
Practice dissolving solids and reading thermometers quickly
Appeared in 2024 — may cycle back after titration-heavy run

Priority Practice #4: Data Presentation

Always create organized data tables — this is heavily graded
Label all columns with units
Show replicate trials
For qualitative: use grid format (unknown × reagent)
Write clear, concise justifications — no rambling

Strategy: Time Management (90 min total)

Read both problems first (~3 min)
Start with whichever problem you're more confident about
Qualitative labs can be faster — do them first if you're quick
Budget ~40 min per problem + 10 min buffer
Get safety approval early — raise hand immediately after writing plan

Predicted Future Topics

Quantitative: Colorimetry/Beer's law (without spectrophotometer), redox titration (permanganate) — kinetics (2016) and gravimetric (2015, 2017) have appeared but not recently
Qualitative: Redox reactions, coordination chemistry colors — chromatography (2018) and polymer chemistry (2019, 2023) may cycle back

Resources

WEBINAR2022ACS Webinar: Preparing for USNCO Part III (for Students)▶

Recorded March 10, 2022 (44 min) — Presenters: Dr. Steve Lantos (Part III Task Force Leader), Dr. Kelly Slunt (Part III Task Force Chair), Dr. Michael Bruno (NC School of Science & Math). Published by ACS Pressroom.

The grading team scores your plan, data, and conclusions — not just the final answer. Partial credit is awarded generously for clear reasoning.

What Graders Look For

Reasonable, step-by-step experimental plan — easy to follow; specify equipment, volumes, concentrations
Legible, organized data tables — label columns with units; this is heavily weighted
Multiple trials — 2-3 trials expected; chemicals are portioned for this, so don't use everything in trial 1
Error analysis & assumptions — mention sources of error even if not explicitly asked; it shows depth
Conclusions based on YOUR data — not what "should" happen theoretically; the grading team checks that your conclusion follows from what you actually observed

Procedure Writing Tips

Include specific equipment names and graduations (e.g., "25 mL graduated cylinder")
State reagent amounts and concentrations explicitly
Indicate that multiple trials will be performed
Note qualitative observations you'll watch for (color changes, bubbling, temperature)
Example: "Measure 25.0 mL of HCl using a 50 mL graduated cylinder into a 250 mL flask. Add 2-3 drops phenolphthalein. Titrate with NaOH dropwise, recording drops to endpoint."

Problem Categories (from ACS)

Titration — determining unknown solution concentration (micro-scale, drop-counting)
Unknown ID — mystery powders, mystery solutions (qualitative analysis)
Quantifying substances — measuring amount of a gas or precipitate produced
Calorimetry — specific heat, heat of reaction, or heat of vaporization
Stoichiometry — determining a predicted amount of a substance (can stand alone, not just titration)
Descriptive chemistry — conclusions drawn from observations of chemical reactions

Multiple Valid Approaches

These are open-ended problems — creative approaches are accepted and graded fairly
The posted rubric shows the most common approach, not the only correct one
You may change your procedure mid-experiment — just get the proctor to sign off again for safety
Clearly document what you actually did so the grading team can follow your reasoning

"I can't emphasize enough that you should read through both problems before you start." — Dr. Michael Bruno

90-Minute Strategy

First 3-5 minutes: Read BOTH problems completely; devise a mental plan for each
Write both plans before starting any experiment — proctor checks safety (not correctness)
Start with the easier/faster problem — students who focus on one and forget the other is the #1 pitfall
Interleave problems: While waiting for drying, crystallization, or reactions in Problem 1, work on Problem 2
Watch the clock: Make mental notes of what's left; ensure you complete at least one problem fully
Budget 10-15 minutes at the end for calculations, conclusions, and write-up — students frequently run out of time here

Common Time Traps

Spending too long on one problem and neglecting the other
Not leaving time for calculations and write-up
Using all chemicals in trial 1 and needing a refill (costs time)
Not planning for reaction/drying wait times

Common Equipment (All Microscale)

Beral pipettes (primary volume delivery — practice with these!)
Syringes (for more precise volume measurement)
Well plates (for microscale reactions and qualitative testing)
Test tubes, small flasks, graduated cylinders
Electronic balances (shared, 0.01 g precision)
Hot plates (no open flames ever)
Thermometers, voltage meters, filter paper, weighing paper, stirring devices
pH indicator strips (used in place of pH meters)

Safety Rules

Safety goggles and closed-toe shoes are mandatory
Hair must be tied back
Lab aprons may be provided or required
Proctor must initial your plan before you begin — they check for safety only
Chemicals are kept at safe concentrations (typically ≤ 3 M); no highly toxic reagents
All waste goes into labeled containers, not down the sink

Important Logistics

One refill of any substance allowed, no penalty — but plan ahead to avoid needing it
Use pencil — easier to correct than pen; same pencil as Parts I & II
Only what's written in the provided answer space is graded — no extra pages
Clean up your station when finished — dirty glassware in one pile, waste in containers

"We strive to involve problems that involve consumer products to instill in students an excitement about how chemistry can be found in our everyday lives." — Dr. Kelly Slunt

Consumer Products Used in Past Exams

Product	Chemistry Application
Vinegar	Acid source for titrations, reactions with carbonates
Yellow Mustard	Turmeric as natural pH indicator for titration endpoint
Bleach	NaOCl oxidizer for kinetics / redox
Hydrogen Peroxide	Decomposition kinetics, calorimetry (ΔH)
Milk	Ca²⁺/Mg²⁺ content via EDTA titration
Drano®	NaOH-based drain cleaner analysis
7-Up®	Acid content / carbonation (CO₂ gas)
Grape Juice	Natural pH indicator (anthocyanins)
Potatoes	Catalase enzyme source / starch testing
Yeast	Biological catalyst for decomposition
Rubbing Alcohol	Solvent properties, evaporation
Ink Pens	Chromatography (dye separation)
HDPE, LDPE plastics	Polymer density / property identification
Galvanized Washers	Zinc coating analysis, metal reactivity
US coins, Japanese yen	Metal composition, electrochemistry
Plastic Wall Screws	Polymer identification
Alka-Seltzer® (2017)	NaHCO₃ / citric acid mass % by CO₂ loss
Kool-Aid® (2018)	Dye chromatography separation
TUMS® (2022)	CaCO₃ back-titration for antacid analysis
Diapers (2019)	Sodium polyacrylate polymer absorption

Lab Bench Setup (2017 Exam)

Sample lab bench setup from 2017 USNCO Part III exam showing three zones: Problem 1 on left, shared items in center, Problem 2 on right

Photo Credit: Dr. K. Slunt — from ACS Webinar (March 2022)

Materials are physically separated into three zones on the bench:
Left: Problem 1 specific equipment and chemicals
Center: Shared items (balance, wash bottle, deionized water, paper towels, waste container)
Right: Problem 2 specific equipment and chemicals
You may work on either problem in any order and switch freely between them

Selected Q&A from the Webinar

Question	Answer
Is there a penalty for requesting a chemical refill?	No penalty for one refill. Beyond that, use what you have.
Should we mention possible errors even if not asked?	Yes — always mention error sources and assumptions. Shows depth and can earn partial credit even if your answer is off.
Can we use extra pages for our answer?	No. Only the provided answer space is graded.
Is there a penalty for breaking glassware?	No, but try to avoid it. Extra glassware should be available.
How important is the quantitative result?	An acceptable range is defined; some error is expected. But the grading team weights your plan, data, and reasoning alongside the final number.
How many trials are recommended?	2-3 trials are reasonable if you're economical with chemicals.
Are more efficient methods given more points?	No — efficiency isn't scored. Clarity of plan, quality of data, and soundness of conclusions matter more.
Will toxic or dangerous chemicals be used?	No highly toxic chemicals (no chromates, lead, etc.). Acids/bases kept at low concentrations (≤ 3 M). All suitable for a high school lab setting.
Does the proctor check if my plan will work?	No — the proctor only checks for safety. They will not tell you if your approach will succeed.
Can I change my procedure mid-experiment?	Yes — just have the proctor sign off again for safety and note the changes.

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