Explain why a solution of sodium ethanoate (CH₃COONa) has a pH greater than 7 at 25 °C.

IB DP Chemistry Standard Level (2023 syllabus) — R3.1 Proton transfer reactions · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Sodium ethanoate fully dissociates in water:
CH₃COONa → CH₃COO⁻ + Na⁺
The ethanoate ion is a Brønsted–Lowry base and accepts a proton from water:
CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻
This generates hydroxide ions, so [OH⁻] > [H⁺] and the solution is basic (pH > 7).

Examiner tips

  • Show the dissociation step first, then the base reaction with water, and finish with the pH conclusion.
  • Use the correct ion names (ethanoate, ethanoic acid) and mention the equilibrium.
  • Mention that Na⁺ is inert and does not affect pH.

Common mistakes

  • Confusing CH₃COOH with CH₃COO⁻ as the species that reacts with water.
  • Forgetting to state that Na⁺ is spectator and does not change pH.
  • Using the wrong direction of the equilibrium or omitting the OH⁻ production.

Mark scheme (4 marks)

  1. Sodium ethanoate fully dissociates / ionises in water to give ethanoate ions (CH₃COO⁻) and sodium ions.
  2. The ethanoate ion acts as a Brønsted–Lowry base and accepts a proton from water.
  3. This produces ethanoic acid (CH₃COOH) and hydroxide ions (OH⁻), establishing the equilibrium: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻.
  4. The concentration of OH⁻ exceeds that of H⁺ / H₃O⁺, so the solution is basic and pH > 7.

Key terms in this question

pH

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