AQA A-Level Chemistry Paper 3, June 2018: Question 31

1 mark · Medium difficulty · Multiple Choice

Identify which pair of equimolar solutions produces the greatest mass of solid precipitate when equal volumes are mixed.

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Question

Question 31 asks: 'Some 1.0 mol dm^-3 solutions were mixed using equal volumes of each solution. Which pair of solutions would give the greatest mass of solid?' Options: A: Ba(OH)2 and MgCl2; B: Ba(OH)2 and MgSO4; C: Ba(OH)2 and NaCl; D: Ba(OH)2 and Na2SO4. Each option has an associated selection lozenge.
Question text

31 Some 1.0 mol dm–3 solutions were mixed using equal volumes of each solution.

Which pair of solutions would give the greatest mass of solid?

[1 mark]

A Ba(OH)2 and MgCl2

B Ba(OH)2 and MgSO4

C Ba(OH)2 and NaCl

D Ba(OH)2 and Na2SO4

Mark scheme

Show the mark scheme Mark scheme for question 31 showing the correct answer as B.

31 B

How to answer it

Precipitation and Group 2 Solubility Trends

📋 What this question tests
  • Group 2 Solubility Trends: Hydroxide solubility increases down Group 2 (Mg(OH)₂ is sparingly soluble/insoluble; Ba(OH)₂ is soluble).
  • Sulfate Solubility Trends: Sulfate solubility decreases down Group 2 (MgSO₄ is soluble; BaSO₄ is completely insoluble).
  • Stoichiometry & Mass of Precipitates: Deducing the total mass of solid products formed from equimolar mixtures of ionic compounds.

Question 31 Analysis

Equimolar mixtures of Group 2 and ionic solutions

Mark Scheme Allocation: 1 Mark for selecting option B.

✅ Correct Answer: B

Ba(OH)₂ and MgSO₄ produce two separate insoluble precipitates:

Ba(OH)₂(aq) + MgSO₄(aq) → BaSO₄(s) + Mg(OH)₂(s)

Because both products are insoluble solids, the total mass of solid produced is the sum of both: 1 mol of BaSO₄ (233.4 g) plus 1 mol of Mg(OH)₂ (58.3 g) = 291.7 g.

💡 Key Knowledge: Group 2 Solubility

  • Hydroxides: Solubility increases down the group.
    • Mg(OH)₂ → Sparingly soluble (forms a white solid precipitate)
    • Ba(OH)₂ → Soluble
  • Sulfates: Solubility decreases down the group.
    • MgSO₄ → Soluble
    • BaSO₄ → Completely insoluble (used in barium meals / sulfate tests)
  • Group 1 salts and chlorides: NaCl and BaCl₂ are highly soluble.

📐 Step-by-Step Comparison (Assuming 1 dm³ of each 1.0 mol dm⁻³ solution)

Equal volumes of 1.0 mol dm⁻³ mean exactly 1.0 mol of each reactant is available.

  1. Option A: Ba(OH)₂ + MgCl₂
    Reaction: Ba(OH)₂(aq) + MgCl₂(aq) → Mg(OH)₂(s) + BaCl₂(aq)
    Solid formed: 1.0 mol of Mg(OH)₂
    Mass = 1.0 × (24.3 + 34.0) = 58.3 g
  2. Option B: Ba(OH)₂ + MgSO₄
    Reaction: Ba(OH)₂(aq) + MgSO₄(aq) → BaSO₄(s) + Mg(OH)₂(s)
    Solid formed: 1.0 mol BaSO₄ + 1.0 mol Mg(OH)₂
    Mass = (137.3 + 32.1 + 64.0) + (24.3 + 34.0) = 233.4 g + 58.3 g = 291.7 g
  3. Option C: Ba(OH)₂ + NaCl
    Reaction: No insoluble combination (BaCl₂ and NaOH are both soluble).
    Solid formed: 0 g
  4. Option D: Ba(OH)₂ + Na₂SO₄
    Reaction: Ba(OH)₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2 NaOH(aq)
    Solid formed: 1.0 mol BaSO₄
    Mass = 1.0 × 233.4 = 233.4 g

❌ Common Errors & Pitfalls

  • Picking Option D: Students often recognize BaSO₄ as a heavy insoluble solid (Mr = 233.4) and choose D without noticing that in B, Mg(OH)₂ also precipitates.
  • Mixing up solubility trends: Confusing the trends for Group 2 sulfates and hydroxides (e.g., thinking MgSO₄ is insoluble or Ba(OH)₂ is insoluble).
  • Assuming only one precipitate forms: Overlooking that both cation-anion swap products in a double displacement can be insoluble at the same time.

🧠 Exam Technique: MCQ Elimination

  • Quick Rule: When asked for the "greatest mass of solid", immediately check if any reaction produces two precipitates instead of just one.
  • Check spectator ions: Na⁺ and Cl⁻ ions almost always remain dissolved in aqueous solution, so combinations involving them (C and D) will never produce more than one precipitate.

Topics

Inorganic Chemistry · Physical Chemistry · 3.2.2 Group 2, The Alkaline Earth Metals · 3.1.2 Amount of Substance

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.