A buffer solution is prepared by dissolving sodium ethanoate in excess ethanoic acid. Explain how this buffer solution resists a change in pH when a small amount of hydrochloric acid is added to it.

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).

Ethanoic acid is a weak acid that partially dissociates in aqueous solution. Its conjugate base, the ethanoate ion, is present in relatively high concentration in the buffer solution.

Model answer (5 marks)

Hydrochloric acid supplies H⁺ (or H₃O⁺) ions. The ethanoate ions (CH₃COO⁻) in the buffer react with these added H⁺ to form ethanoic acid (CH₃COOH). This reaction removes the excess H⁺, keeping the concentration of free H⁺ – and therefore the pH – essentially unchanged. Consequently the buffer resists any significant change in pH.

Examiner tips

  • Mention H⁺ source (HCl) and the reaction CH₃COO⁻ + H⁺ → CH₃COOH. Show that this removes added H⁺, keeping [H⁺] constant. Use terms ‘buffer’, ‘conjugate base’, ‘weak acid’.
  • Keep answer concise – 5 marks, so one clear point per mark. Avoid extra explanation of equilibrium constants.

Common mistakes

  • Failing to identify the reaction between CH₃COO⁻ and H⁺.
  • Using incorrect species (e.g., writing CH₃COOH + H⁺ → CH₃COO⁻).
  • Not linking the reaction to the maintenance of constant [H⁺] and pH.

Mark scheme (5 marks)

  1. Hydrochloric acid is a source of hydrogen ions (H⁺ / H₃O⁺)
  2. The ethanoate ions (CH₃COO⁻) in the buffer react with / remove the added H⁺ ions
  3. This reaction produces more ethanoic acid (CH₃COOH), reforming the weak acid
  4. The concentration of H⁺ ions in solution does not change significantly / remains approximately constant
  5. Therefore the pH remains approximately constant / does not change significantly

Key terms in this question

buffer solution · pH

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