A student has two acid solutions: hydrochloric acid and carbonic acid. Both solutions have the same concentration of 0.1 mol/dm³. The student measures the pH of each solution and finds that hydrochloric acid has a lower pH than carbonic acid at the same concentration. Explain why the two solutions have different pH values, even though they have the same concentration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Hydrochloric acid is a strong acid. Carbonic acid is a weak acid.
Model answer (5 marks)
Hydrochloric acid is a strong acid and is fully dissociated in water, giving 0.1 mol dm⁻³ of H⁺.
Carbonic acid is a weak acid and only partially dissociates, so the H⁺ concentration is less than 0.1 mol dm⁻³.
Because the H⁺ concentration from HCl is higher, its pH is lower.
Thus the lower pH of the hydrochloric acid solution is due to its greater hydrogen‑ion concentration compared with the weak acid at the same overall concentration.
Carbonic acid is a weak acid and only partially dissociates, so the H⁺ concentration is less than 0.1 mol dm⁻³.
Because the H⁺ concentration from HCl is higher, its pH is lower.
Thus the lower pH of the hydrochloric acid solution is due to its greater hydrogen‑ion concentration compared with the weak acid at the same overall concentration.
Examiner tips
- Show the dissociation difference first, then link to H⁺ concentration, then to pH.
- Use the exact terms ‘strong acid’, ‘weak acid’, ‘fully dissociated’, ‘partially dissociated’, and ‘hydrogen ion concentration’.
- Remember pH = –log[H⁺] – a higher [H⁺] gives a lower pH.
Common mistakes
- Confusing the concentration of the acid with the concentration of H⁺ produced.
- Using ‘more acidic’ instead of ‘lower pH’. Failing to mention that HCl is fully dissociated while H₂CO₃ is only partially dissociated.
Mark scheme (5 marks)
- Hydrochloric acid is fully / completely dissociated (ionised) in water
- Carbonic acid is only partially / incompletely dissociated (ionised) in water
- Hydrochloric acid produces a higher concentration of hydrogen ions (H⁺) than carbonic acid at the same concentration
- pH is lower when the concentration of hydrogen ions is higher
- Therefore hydrochloric acid has a lower pH because it has a greater hydrogen ion concentration than carbonic acid at the same overall concentration
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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