AQA A-Level Chemistry Paper 3, June 2025: Question 19

1 mark · Easy difficulty · Multiple Choice

Identify which Period 3 oxide produces a solution with the lowest pH when added to water.

Practise this question

Question

Question 19 asks: '0.100 mol of each of the oxides listed is added to separate 1.00 dm3 samples of water. Which oxide produces the solution with the lowest pH?' The options are A: MgO, B: Al2O3, C: SO2, and D: SO3, each with an oval selection button.
Question text

19 0.100 mol of each of the oxides listed is added to separate 1.00 dm3 samples of

water.

Which oxide produces the solution with the lowest pH?

[1 mark]

A MgO

B Al2O3

C SO2

D SO3

Mark scheme

Show the mark scheme Mark scheme for question 19 shows the correct answer is D (SO3), worth 1 mark categorized as AO1.

19 D 1 (AO1) SO3

How to answer it

Period 3 Oxides: Acidity and pH in Aqueous Solution

📋 What this question tests
  • Reactions of Period 3 oxides (MgO, Al₂O₃, SO₂, SO₃) with water.
  • Acid-base character of Period 3 oxides across the period (basic → amphoteric → acidic).
  • Relative acid strength of sulfur oxides in solution: sulfuric(IV) acid (H₂SO₃) vs sulfuric(VI) acid (H₂SO₄).
  • Connecting acid strength to numerical pH values.

Question 19

0.100 mol of each oxide added to 1.00 dm³ of water — Lowest pH

✅ Correct Answer

D — SO₃

Award [1 mark] for selecting option D.

SO₃ reacts vigorously with water to form sulfuric(VI) acid, H₂SO₄, a strong diprotic acid which fully dissociates its first proton, giving a pH around 0 to 1 (the lowest pH among all choices).

💡 Key Knowledge: Period 3 Oxides with Water

  • MgO (Basic): Sparingly soluble; forms weakly alkaline Mg(OH)₂ with pH ≈ 9 .
    Equation: MgO(s) + H₂O(l) ⇌ Mg(OH)₂(aq)
  • Al₂O₃ (Amphoteric): Insoluble in water due to very high lattice enthalpy; pH ≈ 7 .
    Does not react with water.
  • SO₂ (Acidic): Dissolves to form sulfuric(IV) acid (H₂SO₃), a weak acid; pH ≈ 2 to 3 .
    Equation: SO₂(g) + H₂O(l) ⇌ H₂SO₃(aq)
  • SO₃ (Acidic): Reacts exothermically to form sulfuric(VI) acid (H₂SO₄), a strong acid; pH ≈ 0 to 1 .
    Equation: SO₃(g) + H₂O(l) → H₂SO₄(aq)

📐 pH Comparison for the Acidic Oxides

Both oxides were added at 0.100 mol into 1.00 dm³ of water (concentration = 0.100 mol dm⁻³):

  1. For SO₃:
    SO₃(g) + H₂O(l) → H₂SO₄(aq)
    [H₂SO₄] = 0.100 mol dm⁻³
    H₂SO₄ is a strong acid that completely dissociates:
    H₂SO₄ → H⁺ + HSO₄⁻
    [H⁺] ≥ 0.100 mol dm⁻³
    pH = -log₁₀[H⁺] = -log₁₀(0.100) = 1.00 (further dissociation of HSO₄⁻ makes it even lower, around 0.96).
  2. For SO₂:
    SO₂(g) + H₂O(l) ⇌ H₂SO₃(aq)
    H₂SO₃ is a weak acid (dissociates only slightly).
    [H⁺] << 0.100 mol dm⁻³, so pH is significantly higher (typically 2 – 3).

🧠 Exam Technique: Elimination Strategy

  • Step 1: Check the question goal. "Lowest pH" means the most acidic solution (highest [H⁺]).
  • Step 2: Eliminate alkaline/neutral oxides immediately. MgO gives an alkali (pH 9) and Al₂O₃ is insoluble (pH 7). Eliminate A and B at a glance.
  • Step 3: Distinguish between the two acids. SO₂ forms H₂SO₃ (weak acid, partially dissociates), while SO₃ forms H₂SO₄ (strong acid, fully dissociates).
  • Higher oxidation state of S in SO₃ (+6 vs +4 in SO₂) polarises the O-H bond more, making H₂SO₄ a much stronger acid!

❌ Common Misconceptions & Traps

  • Confusing "lowest pH" with "least acidic": Some students mistakenly associate low pH numbers with weak acids or neutrality. Remember: pH = -log₁₀[H⁺] , so a lower pH means a higher [H⁺] and greater acidity.
  • Confusing SO₂ and SO₃ products: A very common error is thinking SO₂ produces H₂SO₄. Remember:
    • SO₂ (+4) → H₂SO₃ (sulfurous / sulfuric(IV) acid, weak)
    • SO₃ (+6) → H₂SO₄ (sulfuric / sulfuric(VI) acid, strong)
  • Assuming Al₂O₃ dissolves: Al₂O₃ is amphoteric, meaning it reacts with hot concentrated acids and bases, but it is completely insoluble in pure water. Adding it to water leaves the pH essentially unchanged at 7.

Topics

Inorganic Chemistry · 3.2.4 Properties of Period 3 Elements and Their Oxides

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.