A student has two solutions: one containing sulfuric acid and one containing ethanoic acid, both at the same concentration. Explain why the sulfuric acid solution has a lower pH than the ethanoic acid solution.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Sulphuric acid is a strong acid, so it is completely ionised in water and releases nearly all of its H⁺ ions. Ethanoic acid is a weak acid and only partially ionises, releasing only a small amount of H⁺. Therefore the sulphuric acid solution has a higher concentration of H⁺ ions and a lower pH.
Examiner tips
- Use the terms ‘strong acid’ and ‘weak acid’ explicitly.
- Explain the difference in ionisation (complete vs partial).
- Show that higher [H⁺] gives lower pH.
Common mistakes
- Confusing sulphuric with sulphuric acid’s diprotic nature – only the first proton matters for pH at equal concentration.
- Using ‘more acidic’ instead of ‘lower pH’ or omitting the ionisation explanation.
Mark scheme (4 marks)
- Sulfuric acid is a strong acid
- Ethanoic acid is a weak acid
- Sulfuric acid is completely ionised in aqueous solution / releases (nearly) all of its H+ ions
- Ethanoic acid is only partially ionised / releases only a small number of H+ ions, so sulfuric acid produces a higher concentration of H+ ions giving a lower pH
Key terms in this question
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →