A student adds a small volume of dilute hydrochloric acid to a buffer solution made from methanoic acid (HCOOH) and sodium methanoate (HCOONa). Explain why the pH of the buffer solution remains approximately constant.

Edexcel A-Level Chemistry (9CH0) — 11.1 pH, Ka and buffer solutions · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

1. The buffer contains a large reservoir of methanoate ions (HCOO⁻) from the sodium methanoate.
2. The added H⁺ ions react with the methanoate ions.
3. This reaction forms methanoic acid (HCOOH), so H⁺ ions are removed from the solution.
4. There is also a large reservoir of undissociated methanoic acid (HCOOH) present.
5. Because the change in [H⁺] is very small, the pH remains approximately constant.

Examiner tips

  • Use the word ‘react’ to show the neutralisation step. Mention the large reservoirs of both HCOO⁻ and HCOOH. Explain that the small change in [H⁺] keeps the pH stable. Keep the answer concise and use the exact terms from the mark scheme.

Common mistakes

  • Forgetting to mention the methanoate ions as the base component. Failing to state that H⁺ ions are removed by forming HCOOH. Using vague language such as ‘buffer keeps pH stable’ without the mechanistic detail.

Mark scheme (5 marks)

  1. The buffer contains a large reservoir of methanoate ions (HCOO⁻) from the sodium methanoate
  2. The added H⁺ ions react with / are removed by the methanoate ions
  3. This forms methanoic acid (HCOOH), so H⁺ ions are not allowed to accumulate
  4. There is also a large reservoir of undissociated methanoic acid (HCOOH) present
  5. Because the change in [H⁺] is very small, the pH remains approximately constant

Key terms in this question

buffer solution · pH

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