AQA A-Level Chemistry Paper 3, 2023: Question 20

1 mark · Easy difficulty · Multiple Choice

Buffer Solution Formation This question examines the identification of a pair of solutions that, when mixed, form a buffer solution. A buffer maintains a relatively constant pH when small amounts of acid or base are added.

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Question

AQA A-Level Chemistry Paper 3, 2023: Question 20
Question text

20 Equal volumes of pairs of solutions are mixed.

Which pair forms a buffer solution?

[1 mark]

A ammonia and ammonium chloride

B ammonia and methylamine

C ethanoic acid and methanoic acid

D hydrochloric acid and sodium hydroxide

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 3, 2023: Question 20

20 A 1 (AO1) ammonia and ammonium chloride

How to answer it

Buffer Solution Formation

Question

Equal volumes of pairs of solutions are mixed. Which pair forms a buffer solution?

  • A: Ammonia and ammonium chloride
  • B: Ammonia and methylamine
  • C: Ethanoic acid and methanoic acid
  • D: Hydrochloric acid and sodium hydroxide

Correct Answer: A

The correct pair is ammonia and ammonium chloride . This mixture forms a buffer solution because it contains a weak base (ammonia) and its conjugate acid (ammonium ion, NH4+, from ammonium chloride).

Explanation

A buffer solution resists changes in pH when small amounts of acid or base are added. It typically consists of:

  • A weak acid and its conjugate base, or
  • A weak base and its conjugate acid.

In this case, ammonia (NH3) is a weak base, and ammonium chloride (NH4Cl) provides the conjugate acid (NH4+).

Teacher Tip: Emphasise that a buffer must include a weak acid/base and its conjugate pair. Strong acids or bases, like HCl or NaOH, cannot form buffers on their own.

Why Other Options Are Incorrect

  • B: Ammonia and methylamine are both weak bases. Without a conjugate acid or acid/base pair, this mixture cannot form a buffer.
  • C: Ethanoic acid and methanoic acid are both weak acids. Similarly, without a conjugate base, this combination cannot form a buffer.
  • D: Hydrochloric acid (strong acid) and sodium hydroxide (strong base) neutralise each other rather than forming a buffer.

Final Answer

A: Ammonia and ammonium chloride

Topics

Physical Chemistry · 3.1.12 Acids and Bases

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.