A student prepares two separate solutions: a 0.1 mol/dm³ solution of sulfuric acid and a 0.1 mol/dm³ solution of ethanoic acid. Explain why the sulfuric acid solution has a lower pH than the ethanoic acid solution, despite both solutions having the same concentration.

AQA A-Level Chemistry (7405) — 3.1.12 Acids and bases (A-Level only) · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Sulphuric acid is a strong acid, so it completely dissociates in water to give H⁺ (or H₃O⁺) ions.

Ethanoic acid is a weak acid, so it only partially dissociates and an equilibrium is established.

Because the sulphuric acid solution produces a higher concentration of H⁺ ions than the ethanoic acid solution, its pH is lower.

Examiner tips

  • Use the command word ‘Explain’ – give a clear, concise reason. Mention that strong acids fully dissociate, weak acids only partially. Show the link between H⁺ concentration and pH (pH = –log[H⁺]).

Common mistakes

  • Confusing the concentration of the acid with the concentration of H⁺ ions. Forgetting to state that sulphuric acid is a strong acid and ethanoic acid is weak. Using the wrong spelling of ‘sulphuric’ (UK spelling required).

Mark scheme (5 marks)

  1. Sulfuric acid is a strong acid
  2. Ethanoic acid is a weak acid
  3. Strong acids completely / fully dissociate in water to produce hydrogen ions (H⁺ / H₃O⁺)
  4. Weak acids only partially dissociate / establish an equilibrium in solution, so fewer H⁺ ions are produced
  5. The greater concentration of H⁺ ions in the sulfuric acid solution results in a lower pH

Key terms in this question

pH

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