Explain why a solution of ethanoic acid has a higher pH than a solution of hydrochloric acid of the same concentration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ethanoic acid and hydrochloric acid are both acids, but they behave differently in aqueous solution.
Model answer (5 marks)
Hydrochloric acid is a strong acid and therefore fully dissociates in water, giving a high concentration of H⁺ (or H₃O⁺) ions.
Ethanoic acid is a weak acid and only partially dissociates, so it produces a lower concentration of H⁺ ions.
Because the concentration of H⁺ ions is lower, the pH of the ethanoic acid solution is higher.
Thus, at the same concentration, ethanoic acid has a higher pH than hydrochloric acid.
Ethanoic acid is a weak acid and only partially dissociates, so it produces a lower concentration of H⁺ ions.
Because the concentration of H⁺ ions is lower, the pH of the ethanoic acid solution is higher.
Thus, at the same concentration, ethanoic acid has a higher pH than hydrochloric acid.
Examiner tips
- State that HCl is a strong acid and fully dissociates; ethanoic acid is weak and partially dissociates. Explain that fewer H⁺ ions give a higher pH. Use the term ‘pH’ and ‘hydrogen ion concentration’ to show understanding.
Common mistakes
- Confusing the terms ‘strong’ and ‘weak’ or saying both acids dissociate fully. Failing to mention that the lower H⁺ concentration leads to a higher pH. Using ‘more H⁺’ for the weak acid instead of ‘fewer H⁺’.
Mark scheme (5 marks)
- Hydrochloric acid is a strong acid and fully/completely dissociates (ionises) in aqueous solution
- Ethanoic acid is a weak acid and only partially/incompletely dissociates (ionises) in aqueous solution
- At the same concentration, ethanoic acid produces fewer hydrogen ions (H⁺ / H₃O⁺) in solution than hydrochloric acid
- A lower concentration of H⁺ ions means a higher pH
- Therefore, at the same concentration, ethanoic acid has a higher pH than hydrochloric acid because its H⁺ ion concentration is lower
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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