OCR A Level Chemistry A: Practical skills assessed in a written examination (planning, implementing, analysis and evaluation) (H432) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

Coverage: section 1.1 (1.1.1 Planning to 1.1.4 Evaluation) of the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017. The section has no tiers and is assessed in the written papers.

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Questions

1. Units.

(a) State the units of a rate of reaction measured as change in concentration per second. [1] (b) A rate equation is rate = k[X]²[Y]. Give the units of k. [1] (c) Convert 18.6 cm³ into dm³. [1]

2. 2.145 g of ethanedioic acid dihydrate (Mr = 126.0) is dissolved and made up to 250.0 cm³ in a volumetric flask. Calculate the concentration, giving your answer to an appropriate number of significant figures. [2]

3. In a titration, the initial burette reading is 1.35 cm³ and the final reading is 19.85 cm³. Each reading has an uncertainty of ±0.05 cm³.

(a) Calculate the titre and its percentage uncertainty. [2] (b) Suggest one change to the procedure that would halve this percentage uncertainty. [1]

4. A student investigates how temperature affects the time taken for acidified potassium manganate(VII) to be decolourised by excess ethanedioic acid.

(a) Identify the independent and dependent variables. [1] (b) State three variables that must be controlled, and how each would be controlled. [3]

5. A student records this table while collecting gas.

Time (seconds)	Volume
30	12 cm³
60	21.5 cm³
90	28 cm³

State three ways the table should be improved. [3]

6. A white solid Z is tested. Test 1: warming with NaOH(aq) gives a gas that turns damp red litmus blue. Test 2: dilute nitric acid gives no effervescence. Test 3: Ba(NO₃)₂(aq) gives a white precipitate.

(a) Identify Z, explaining your reasoning from each test. [3] (b) Explain why the test for sulfate is carried out before any test for halide ions. [1]

7. Titres (cm³): rough 23.10, then 22.55, 22.30, 22.60, 22.50. Choose the titres to use and calculate the mean titre. [2]

8. The initial rate of a reaction is measured at different concentrations of reactant A.

[A] / mol dm⁻³	0.020	0.040	0.060	0.080	0.100
initial rate / mol dm⁻³ s⁻¹	6.20 × 10⁻⁵	1.25 × 10⁻⁴	1.60 × 10⁻⁴	2.50 × 10⁻⁴	3.11 × 10⁻⁴

A graph of initial rate against [A] gives a straight line through the origin.

(a) Identify the anomalous result and explain your choice. [2] (b) Determine the rate constant k from the gradient of the line. [2] (c) Give the units of k. [1] (d) State what the shape of the graph shows about the order with respect to A. [1]

9. On a concentration–time curve for a reactant, a tangent drawn at t = 60 s passes through (0 s, 0.150 mol dm⁻³) and (150 s, 0.000 mol dm⁻³).

(a) Calculate the rate of reaction at 60 s. [2] (b) Explain why the tangent should be extended as far as possible before its gradient is measured. [1]

10. A student finds the volume of hydrogen from 0.060 g of magnesium ribbon with excess hydrochloric acid. She removes the bung of the flask, drops in the ribbon, replaces the bung and collects the gas over water in a 100 cm³ measuring cylinder. She collects 52 cm³. Use Ar(Mg) = 24.3 and a molar gas volume of 24.0 dm³ mol⁻¹.

(a) Calculate the volume of hydrogen expected. [2] (b) Suggest two limitations of the procedure that explain the low volume, and an improvement for each. [4]

11. A student heats hydrated barium chloride, BaCl₂·xH₂O, in a crucible. Each balance reading is ±0.001 g. Use Mr(BaCl₂) = 208.3 and Mr(H₂O) = 18.0.

Reading	Mass / g
empty crucible	18.462
crucible + hydrated salt	20.904
crucible + residue after heating	20.560

(a) Calculate x to two decimal places. [3] (b) Calculate the percentage uncertainty in the mass of water lost. [1] (c) The accepted formula is BaCl₂·2H₂O. Calculate the percentage error in your value of x. [1] (d) Evaluate the result, suggesting the main cause of the difference and an improvement. [3]

12. Each iron tablet is thought to contain about 65 mg of iron as Fe²⁺. You have 0.00400 mol dm⁻³ KMnO₄(aq), dilute sulfuric acid and standard laboratory glassware. Use Ar(Fe) = 55.8 and:

MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O

Plan a titration to find the mass of iron in one tablet. Include quantities, show that your titre will be a suitable size, and explain how the result is calculated. [8]

Answers

1. (a) mol dm⁻³ s⁻¹ [1] (b) k = rate ÷ ([X]²[Y]) = (mol dm⁻³ s⁻¹) ÷ (mol dm⁻³)³, so dm⁶ mol⁻² s⁻¹ [1] (c) 18.6 ÷ 1000 = 0.0186 dm³ [1] Examiner insight: Build units of k from the rate equation each time; a memorised unit for the wrong overall order is unlikely to gain credit.

2. n = 2.145 ÷ 126.0 = 0.017024 mol; c = 0.017024 ÷ 0.2500 [1] = 0.06810 mol dm⁻³ (4 s.f., matching the data) [1] Examiner insight: The data are to 4 significant figures, so 0.068 or 0.0681 shows less precision than the measurements allow.

3. (a) Titre = 19.85 − 1.35 = 18.50 cm³ [1]; uncertainty = 2 × 0.05 = ±0.10 cm³, so 0.10 ÷ 18.50 × 100 = 0.54% [1] (b) Double the titre, for example by using a sample of twice the volume or concentration [1] Examiner insight: Writing the titre as 18.5 cm³ hides the burette’s resolution; record and quote it to two decimal places.

4. (a) Independent: temperature; dependent: time taken to decolourise (both needed) [1] (b) Any three, each with a method: volume and concentration of KMnO₄(aq), by pipette from one stock solution [1]; volume and concentration of ethanedioic acid, likewise [1]; volume and concentration of sulfuric acid, likewise [1]; total volume of the mixture Examiner insight: “Same amount” alone is too vague; name the quantity and say how you would keep it constant.

5. Units belong in the headings, as volume / cm³, not in the cells [1]; time heading should be “time / s” [1]; values in a column need the same number of decimal places, for example 12.0, 21.5, 28.0 [1] Examiner insight: Three different faults are needed; repeating “units are wrong” twice is unlikely to earn two marks.

6. (a) Test 1: ammonia released, so NH₄⁺ present [1]. Test 2: no CO₂, so carbonate absent [1]. Test 3: white BaSO₄, so sulfate present; Z is (NH₄)₂SO₄, ammonium sulfate [1] (b) Sulfate ions could precipitate with Ag⁺ and be mistaken for a silver halide [1] Examiner insight: Link each observation to an ion; a correct formula with no reasoning does not answer an “explain” question.

7. Concordant titres 22.55, 22.60 and 22.50; 22.30 and the rough titre are left out [1]; mean = 67.65 ÷ 3 = 22.55 cm³ [1] Examiner insight: Averaging every value, including the rough titre, gives a mean that does not reflect the concordant results.

8. (a) [A] = 0.060 mol dm⁻³ [1]; the others give rate ÷ [A] ≈ 3.1 × 10⁻³ s⁻¹, this one 2.67 × 10⁻³ s⁻¹, so it lies below the line [1] (b) Gradient from (0, 0) and (0.100, 3.11 × 10⁻⁴) on the line [1]: k = 3.11 × 10⁻⁴ ÷ 0.100 = 3.11 × 10⁻³ [1] (c) s⁻¹ [1] (d) Rate is directly proportional to [A], so the reaction is first order with respect to A [1] Examiner insight: Take gradient points from the drawn line, well apart; using the anomalous point pulls k down.

9. (a) Gradient = (0.000 − 0.150) ÷ (150 − 0) = −1.00 × 10⁻³ [1]; rate = 1.00 × 10⁻³ mol dm⁻³ s⁻¹ [1] (b) A longer line gives larger Δy and Δx, so reading errors are a smaller fraction of each [1] Examiner insight: Rate is quoted as a positive value even though the gradient for a reactant is negative.

10. (a) n(Mg) = 0.060 ÷ 24.3 = 2.47 × 10⁻³ mol = n(H₂) [1]; V = 2.47 × 10⁻³ × 24 000 = 59 cm³ [1] (b) Hydrogen escapes while the bung is off [1]; improvement: hang the ribbon in a small tube inside the sealed flask, or attach it by thread, and release it after sealing [1]. Oxide coating on the ribbon means less magnesium reacts [1]; improvement: clean the ribbon with emery paper before weighing [1] Examiner insight: Each improvement must address the limitation you named; “repeat the experiment” does not fix a systematic loss.

11. (a) Mass of hydrated salt 2.442 g; anhydrous 2.098 g; water 0.344 g [1]; n(BaCl₂) = 2.098 ÷ 208.3 = 0.01007 mol; n(H₂O) = 0.344 ÷ 18.0 = 0.01911 mol [1]; x = 0.01911 ÷ 0.01007 = 1.90 [1] (b) Two readings, so ±0.002 g: 0.002 ÷ 0.344 × 100 = 0.58% [1] (c) (2 − 1.90) ÷ 2 × 100 = 5.0% [1] (d) 5.0% is much larger than the measurement uncertainty (about 0.7% in total), so the error is systematic [1]. The value is low because not all the water was driven off [1]. Heat, cool and reweigh repeatedly until the mass is constant [1] Examiner insight: An evaluation needs the comparison stated; giving 5.0% and 0.58% without saying what they show does not complete it.

12. Weigh five tablets (about 0.33 g of iron) and crush them [1]. Dissolve in dilute sulfuric acid, which supplies the H⁺ the equation needs [1]. Transfer with washings to a 250.0 cm³ volumetric flask, make up to the mark and invert to mix [1]. Pipette 25.0 cm³ into a conical flask [1]. Titrate with KMnO₄ from a burette until the first permanent pale pink colour [1]. Repeat to concordant titres within 0.10 cm³ and average them [1]. Estimate: 5 × 0.065 = 0.325 g, or 5.82 × 10⁻³ mol Fe²⁺; 5.82 × 10⁻⁴ mol in 25.0 cm³; MnO₄⁻ = 1.16 × 10⁻⁴ mol, a titre of about 29 cm³, suitable for a 50 cm³ burette [1]. Mass of Fe per tablet = mean titre (dm³) × 0.00400 × 5 × 10 × 55.8 ÷ 5 [1] Examiner insight: Planning answers gain credit for stated quantities and a reason for each; “add some acid” or “use a titration” alone is too vague.

Where marks are usually lost

• Controlled variables named without saying how they are kept constant.
• Units in table cells, or headings with no unit.
• ±0.05 cm³ used for a titre from two readings.
• Rounding below the precision of the data.
• The rough or a non-concordant titre included in a mean.
• A gradient from an anomalous point, or from close points.
• A negative rate quoted from a reactant’s tangent.
• Limitations with no matching improvement.
• Percentage error and uncertainty calculated but not compared.
• Plans with no quantities, or an unsuitable titre size.

Next steps

• Practical skills revision notes
• Practical skills study guide
• Amount of substance practice
• OCR A Level Chemistry hub
• H432 printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017, published by OCR – Module 1, section 1.1 Practical skills assessed in a written examination (1.1.1 to 1.1.4).