AQA A-Level Chemistry Paper 3, June 2022: Question 15

Predicting Solution pH After Mixing Equal Volumes

📌 What this question tests

This question evaluates your understanding of stoichiometry, acid-base titrations, and salt hydrolysis at 298 K:

Recognising that mixing equal volumes of equimolar solutions yields an exact 1 : 1 molar ratio.
Distinguishing between strong/weak acids and strong/weak bases.
Predicting whether the resulting salt solution undergoes hydrolysis to form an acidic, neutral, or alkaline solution (pH > 7).

Question 15

Identifying the Mixture that Produces pH > 7

Multiple Choice (1 Mark) — AO3 Analysis and Evaluation

✅ Correct Answer: B

KOH and CH₃COOH

When equimolar amounts of potassium hydroxide (strong base) and ethanoic acid (weak acid) are mixed, they neutralise completely to form a solution of potassium ethanoate ( CH₃COOK ):

CH₃COOH(aq) + KOH(aq) → CH₃COOK(aq) + H₂O(l)

The ethanoate ion hydrolyses water to release hydroxide ions:

CH₃COO⁻(aq) + H₂O(l) ⇌ CH₃COOH(aq) + OH⁻(aq)

Since [ OH⁻ ] > [ H⁺ ], the resulting solution has a pH > 7.

📐 Step-by-Step Option Elimination

Because volumes and concentrations are identical, exactly equal moles ( n = c × V ) of each species are mixed:

Option A (HCOOH + HCOOK): Mixture of a weak acid and its conjugate base. This forms an acidic buffer, so pH < 7.
Option B (KOH + CH₃COOH): Strong base + weak acid in a 1 : 1 ratio. Full equivalence is reached. The salt formed hydrolyses to give an alkaline solution, so pH > 7.
Option C (NH₃ + HCl): Weak base + strong acid in a 1 : 1 ratio forms ammonium chloride ( NH₄Cl ). The NH₄⁺ ion hydrolyses releasing H⁺ , so pH < 7.
Option D (NH₄Cl + KCl): KCl is neutral, but NH₄Cl is acidic due to ammonium ion dissociation, so pH < 7.

💡 Key Knowledge: Salt Hydrolysis at Equivalence

Strong Acid + Strong Base: Neutral salt (e.g. NaCl), pH = 7.
Weak Acid + Strong Base: Basic salt (e.g. CH₃COOK), pH > 7 because the conjugate base reacts with water:
A⁻ + H₂O ⇌ HA + OH⁻
Strong Acid + Weak Base: Acidic salt (e.g. NH₄Cl), pH < 7 because the conjugate acid donates a proton to water:
BH⁺ + H₂O ⇌ B + H₃O⁺

❌ Common Student Errors

Assuming neutralisation always produces pH 7: Many students mistakenly believe that 1 mole of any acid neutralised by 1 mole of any base gives exactly pH 7. At equivalence, only strong-strong combinations give pH 7!
Confusing Buffers with Equivalence: In Option A, students often recognise the presence of a salt and acid but forget that equal amounts of weak acid and conjugate salt make an acidic buffer ( pH ≈ pKₐ < 7 ).
Misidentifying Strong vs Weak: Confusing NH₃ as a strong base or HCOOH as a strong acid.

🧠 Exam Technique: 15-Second MCQ Shortcut

For equal volume + equal concentration acid-base questions asking for pH > 7, immediately scan the pairings for:

[Strong Base] + [Weak Acid] → Alkaline Salt (pH > 7)

KOH is the only strong base present in any option, instantly pointing you directly to option B.

Mark Scheme Note: Award 1 mark for Option B. No working is required to be shown on the multiple-choice answer sheet.

AQA A-Level Chemistry Paper 3, June 2022: Question 15

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