A student dissolves citric acid in water and measures the pH. She then dissolves the same amount of hydrochloric acid in the same volume of water and measures the pH. Both solutions have the same concentration. Explain why the two solutions have different pH values, including the terms weak acid and strong acid in your answer.

OCR A-Level Chemistry B: Salters (H433) — 3.2 Physical chemistry · Explain · 5 marks · View as Markdown

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

Citric acid is found in lemons. Hydrochloric acid is used in industry. Both are acids, but their behaviour in water is different.

Model answer (5 marks)

Hydrochloric acid is a strong acid, so it is completely ionised in water, releasing almost all of its H⁺ ions. Citric acid is a weak acid, so it is only partially ionised in water, releasing only a small number of H⁺ ions. Consequently the H⁺ concentration in the hydrochloric acid solution is higher, giving it a lower pH than the citric acid solution.

Examiner tips

  • Use the terms "strong acid" and "weak acid" explicitly.
  • Show the link between ionisation and H⁺ concentration.
  • Mention that the higher H⁺ concentration gives a lower pH.
  • Keep the answer concise and to the point.

Common mistakes

  • Confusing the terms strong and weak acids.
  • Failing to explain that ionisation determines H⁺ concentration.
  • Using vague phrases like "more acidic" without linking to H⁺ concentration.

Mark scheme (5 marks)

  1. Hydrochloric acid is a strong acid
  2. Hydrochloric acid is completely ionised in aqueous solution, releasing (almost) all of its H⁺ ions
  3. Citric acid is a weak acid
  4. Citric acid is only partially ionised in aqueous solution, releasing only a small number of H⁺ ions
  5. The hydrochloric acid solution has a higher concentration of H⁺ ions, so it has a lower pH than the citric acid solution

Key terms in this question

weak acid · strong acid · pH

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