OCR A Level Chemistry A: Qualitative analysis (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.

These questions practise section 3.1.4, Qualitative analysis, of the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017. They test all of learning outcome 3.1.4(a): anion tests in part (i), the ammonium test in part (ii), the order of tests and PAG4 skills. H432 is the full A level. Questions 8 to 10 are longer, combined problems.

Work through the study guide and revision notes before you start. The OCR chemistry hub, printable checklist and free diagnostics cover the wider course.

Data for calculations: Ar values H = 1.0, Li = 6.9, C = 12.0, N = 14.0, O = 16.0, S = 32.1, Cl = 35.5, K = 39.1, Ag = 107.9, Ba = 137.3. Molar gas volume at RTP = 24.0 dm³ mol⁻¹.

Questions

1. A solution of rubidium bromide is acidified with dilute nitric acid. Aqueous silver nitrate is then added.

(a) State what is seen. [1] (b) Give the ionic equation for this reaction, including state symbols. [1] (c) Describe what is seen when dilute aqueous ammonia, and then concentrated aqueous ammonia, is added to the mixture. [2]

2. A student adds aqueous sodium hydroxide to ammonium nitrate at room temperature. She pushes dry red litmus paper into the tube so it rests on the wet glass. Parts of it turn blue.

(a) State two changes the student should make to her method. [2] (b) Explain why each change is needed. [2] (c) Explain why her blue litmus result is not reliable evidence of ammonium ions. [1]

3. The specification gives the order of anion tests as carbonate, then sulfate, then halide. Explain why this order is needed. [4]

4. A student acidifies a solution with dilute nitric acid and adds aqueous barium chloride: a white precipitate forms. He filters, adds aqueous silver nitrate to the filtrate, and sees a white precipitate. He concludes the solution contains sulfate and chloride ions.

Evaluate his conclusions, and suggest one improvement to the method. [3]

5. A white solid, M, is a single compound. The results of tests on M are shown.

Test	Observation
Dilute nitric acid added to a solution of M; gas passed into limewater	effervescence; limewater turns milky
Barium nitrate solution added to the acidified solution	no visible change
Silver nitrate solution added to the same solution	no visible change
A fresh sample of M warmed with NaOH(aq); damp red litmus held at the mouth of the tube	litmus turns blue

Identify M, giving the evidence for each ion, and write the full equation for the reaction of M with dilute nitric acid. [4]

6. A 1.50 g sample of fertiliser contains only ammonium chloride and ammonium sulfate. It is dissolved, acidified with dilute nitric acid and treated with excess barium nitrate solution. The dry precipitate has a mass of 1.590 g.

(a) Calculate the percentage by mass of ammonium sulfate in the fertiliser. [4] (b) Explain why barium nitrate, not barium chloride, is used if the filtrate will later be tested for chloride. [1]

7. A 0.400 g tablet contains lithium carbonate, Li₂CO₃, and inert ingredients that do not react with acid. The tablet is added to excess dilute nitric acid and 96.0 cm³ of carbon dioxide is collected at RTP.

(a) Write the full equation, with state symbols, for the reaction of lithium carbonate with nitric acid. [1] (b) Calculate the percentage by mass of lithium carbonate in the tablet. Answer to three significant figures. [3] (c) State one assumption made in your calculation. [1]

8. Solution Y contains three sodium salts. A student carries out these steps in order.

• Step 1: dilute nitric acid is added until fizzing stops. The gas turns limewater milky.
• Step 2: excess barium nitrate solution is added. A white precipitate forms, which is filtered off.
• Step 3: silver nitrate solution is added to the filtrate. A cream precipitate forms, insoluble in dilute but soluble in concentrated ammonia.
• Step 4: a fresh sample of Y is warmed with NaOH(aq). Damp red litmus held at the mouth of the tube stays red.

(a) Identify the three anions in Y, giving the evidence for each. [3] (b) State what Step 4 shows. [1] (c) Write ionic equations, with state symbols, for the reactions in Steps 1, 2 and 3. [3] (d) Explain why Step 2 had to be done after Step 1 and before Step 3. [2]

9. A 0.714 g sample of a potassium halide, KX, is dissolved, acidified and treated with excess silver nitrate solution. The dry precipitate, AgX, has a mass of 1.127 g.

(a) From the two masses, find Ar of X and name the element. [4] (b) State the colour you expect for this precipitate, and what happens when concentrated aqueous ammonia is added. [2]

10. A colourless solution contains ammonium sulfate and potassium iodide. Describe test-tube tests, in order, to show that it contains NH₄⁺, SO₄²⁻ and I⁻ and no CO₃²⁻. Give reagents, observations and an ionic equation for each positive result. Do not test for K⁺. [8]

Answers

1. (a) A cream precipitate [1] (b) Ag⁺(aq) + Br⁻(aq) → AgBr(s) [1] (c) With dilute ammonia the precipitate does not dissolve [1]; with concentrated ammonia it dissolves [1] Examiner insight: Cream and white look alike in a tube, so describe both ammonia steps separately rather than writing “partly soluble”.

2. (a) Warm the mixture [1]. Use damp red litmus held at the mouth of the tube, not touching the glass [1] (b) Ammonia is very soluble in water; warming drives it out as a gas [1]. Damp paper lets NH₃ dissolve and form OH⁻; keeping it off the glass stops NaOH turning it blue [1] (c) The paper touched glass wet with alkaline NaOH, so the blue may come from NaOH, not ammonia [1] Examiner insight: Part (b) needs the chemistry behind each change (solubility of NH₃, formation of OH⁻), not a repeat of part (a).

3. Carbonate ions would react with Ba²⁺ to form BaCO₃, which is insoluble and white, giving a false positive for sulfate [1]. Carbonate ions would also form a precipitate with Ag⁺ in the halide test [1]. So acid is added first to remove carbonate as CO₂ (and to show whether it is present) [1]. Ag₂SO₄ is insoluble, so sulfate would precipitate with Ag⁺; sulfate must be tested for and removed before the halide test [1] Examiner insight: Name the insoluble compound (BaCO₃, Ag₂SO₄) at each step; “the ions interfere” is too vague to earn much.

4. Sulfate conclusion valid: the solution was acidified, so carbonate is ruled out and the precipitate is BaSO₄ [1]. Chloride conclusion not valid: barium chloride added Cl⁻, so AgCl would form anyway [1]. Improvement: use barium nitrate instead of barium chloride, then filter and add silver nitrate to the filtrate [1] Examiner insight: “Evaluate” needs a separate judgement on each conclusion, with the reason tied to the Cl⁻ in the reagent.

5. Effervescence with a gas that turns limewater milky shows CO₃²⁻ [1]. An alkaline gas (ammonia) on warming with NaOH shows NH₄⁺ [1]. M is ammonium carbonate, (NH₄)₂CO₃ [1] (NH₄)₂CO₃ + 2HNO₃ → 2NH₄NO₃ + CO₂ + H₂O [1] Examiner insight: Use the negative results too: the barium and silver tests rule out sulfate and halides.

6. (a) Mr(BaSO₄) = 233.4; n(BaSO₄) = 1.590 ÷ 233.4 = 0.00681 mol [1]. n((NH₄)₂SO₄) = n(BaSO₄) = 0.00681 mol [1]. Mr((NH₄)₂SO₄) = 132.1; mass = 0.00681 × 132.1 = 0.900 g [1]. Percentage = 0.900 ÷ 1.50 × 100 = 60.0% [1] (b) Barium chloride would add chloride ions to the filtrate, so the chloride measured would be too high [1] Examiner insight: All the BaSO₄ comes from ammonium sulfate; do not treat the whole 1.50 g as one compound.

7. (a) Li₂CO₃(s) + 2HNO₃(aq) → 2LiNO₃(aq) + CO₂(g) + H₂O(l) [1] (b) n(CO₂) = 0.0960 ÷ 24.0 = 0.00400 mol = n(Li₂CO₃) [1]. Mr(Li₂CO₃) = 73.8; mass = 0.00400 × 73.8 = 0.2952 g [1]. Percentage = 0.2952 ÷ 0.400 × 100 = 73.8% [1] (c) Any one: all the CO₂ was collected (none dissolved); no other ingredient released gas; the volume was measured at RTP [1] Examiner insight: Convert 96.0 cm³ to 0.0960 dm³ before dividing by 24.0; a percentage far above 100 signals a unit slip.

8. (a) CO₃²⁻: effervescence with a gas that turns limewater milky in Step 1 [1]. SO₄²⁻: white precipitate with Ba²⁺ in the acidified solution in Step 2 [1]. Br⁻: cream precipitate with Ag⁺, insoluble in dilute but soluble in concentrated ammonia, in Step 3 [1] (b) No ammonium ions are present (no ammonia released) [1] (c) Step 1: CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l) [1]. Step 2: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) [1]. Step 3: Ag⁺(aq) + Br⁻(aq) → AgBr(s) [1] (d) After Step 1, because carbonate had to be removed first or it would have formed white BaCO₃ with Ba²⁺ [1]. Before Step 3, because Ag₂SO₄ is insoluble, so sulfate had to be precipitated and filtered off before silver ions were added [1] Examiner insight: In (a) pair each ion with its own observation so the evidence for each is clear.

9. (a) n(KX) = n(AgX), so 0.714 ÷ (39.1 + A) = 1.127 ÷ (107.9 + A) [1]. 0.714(107.9 + A) = 1.127(39.1 + A), so 77.04 − 44.07 = (1.127 − 0.714)A [1]. A = 32.97 ÷ 0.413 = 79.8 [1]. X is bromine (Ar about 79.9) [1] (b) Cream precipitate [1]; it dissolves in concentrated ammonia [1] Examiner insight: Write the equal-moles expression before rearranging; it is the step that shows your method.

10. Add dilute nitric acid to a sample: no effervescence, so no CO₃²⁻ [1]. Then add barium nitrate solution (not barium chloride, which would add Cl⁻) [1]: white precipitate, showing SO₄²⁻ [1]; Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) [1]. Filter, then add silver nitrate solution to the filtrate: yellow precipitate [1], which does not dissolve in concentrated ammonia, showing I⁻; Ag⁺(aq) + I⁻(aq) → AgI(s) [1]. On a fresh sample, add NaOH(aq) and warm; damp red litmus at the mouth of the tube turns blue, showing NH₄⁺ [1]; NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l) [1] Examiner insight: A plan needs order and reasons, not only reagents: say why nitric acid comes first and why barium nitrate is used.

Where marks are usually lost

• Writing “goes cloudy” instead of naming a precipitate and its colour.
• Treating fizzing alone as proof of carbonate without the limewater result.
• Acidifying with hydrochloric acid before a halide test, as in question 4.
• Using barium chloride when a halide test follows in the same solution.
• Forgetting to warm in the ammonium test, or using dry litmus or letting it touch the tube (question 2).
• Putting (aq) on a precipitate in an ionic equation.
• Leaving a gas volume in cm³ when dividing by 24.0 dm³ mol⁻¹ (question 7).
• Rearranging the KX and AgX mole equation wrongly (question 9).

Next steps

• Tables and method steps: Qualitative analysis revision notes
• Full explanations and worked examples: Qualitative analysis study guide
• Every H432 section in one place: OCR chemistry hub
• Course-wide check: all free 10-minute diagnostics
• Mark off outcomes: H432 checklist
• Gravimetric and gas-volume calculations: Amount of substance questions
• Book a free trial class: /trial/

Official syllabus

OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017, section 3.1.4 Qualitative analysis, learning outcome (a)(i) and (ii), with its additional guidance and PAG4. Published by OCR.