Hydrochloric acid and ethanoic acid are both acids used in laboratories. Explain why hydrochloric acid is described as a strong acid and ethanoic acid is described as a weak acid, and explain how this difference affects the pH of two solutions that have the same concentration.

Edexcel A-Level Chemistry (9CH0) — 12.1 Strong and weak acids · Explain · 5 marks · View as Markdown

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

Hydrochloric acid has a concentration of 0.1 mol/dm³. Ethanoic acid also has a concentration of 0.1 mol³.

Model answer (5 marks)

Hydrochloric acid (HCl) is a strong acid because it completely dissociates in water:
1. HCl → H⁺ + Cl⁻ (100 % ionisation).
2. Ethanoic acid (CH₃COOH) is a weak acid; it only partially dissociates:
3. CH₃COOH ⇌ H⁺ + CH₃COO⁻ (equilibrium, < 10 %).
4. At the same 0.1 mol dm⁻³ concentration, HCl gives a higher [H⁺] than ethanoic acid.
5. The higher [H⁺] gives HCl a lower pH; pH decreases as [H⁺] increases.

Examiner tips

  • Use the word ‘completely’ for strong acids and ‘partially’ for weak acids. Show the ionisation equations. Link higher [H⁺] to lower pH. Mention that pH = –log[H⁺].

Common mistakes

  • Confusing concentration units (mol dm⁻³ vs mol³). Saying both acids dissociate fully. Forgetting to state the effect on pH.

Mark scheme (5 marks)

  1. Hydrochloric acid completely/fully dissociates (ionises) in water
  2. Ethanoic acid only partially/incompletely dissociates (ionises) in water
  3. Hydrochloric acid produces a higher concentration of hydrogen ions (H⁺) than ethanoic acid at the same concentration
  4. Hydrochloric acid therefore has a lower pH than ethanoic acid (at the same concentration)
  5. pH decreases as hydrogen ion concentration increases (linking lower pH to greater H⁺ concentration)

Key terms in this question

strong acid · weak acid · pH

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