A student measures the pH of two separate solutions: a 0.1 mol/dm³ solution of nitric acid and a 0.1 mol/dm³ solution of methanoic acid. The nitric acid has a lower pH than the methanoic acid. 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).
Nitric acid is a strong acid. Methanoic acid is a weak acid.
Model answer (5 marks)
A 0.1 mol dm⁻³ nitric acid solution is a strong acid, so it ionises completely:
HNO₃ → H⁺ + NO₃⁻
giving 0.1 mol dm⁻³ H⁺.
Methanoic acid is weak; it only partially ionises:
CH₃COOH ⇌ H⁺ + CH₃COO⁻
so the [H⁺] produced is much less than 0.1 mol dm⁻³.
Because pH = –log[H⁺], the higher [H⁺] from nitric acid gives a lower pH, while the lower [H⁺] from methanoic acid gives a higher pH.
HNO₃ → H⁺ + NO₃⁻
giving 0.1 mol dm⁻³ H⁺.
Methanoic acid is weak; it only partially ionises:
CH₃COOH ⇌ H⁺ + CH₃COO⁻
so the [H⁺] produced is much less than 0.1 mol dm⁻³.
Because pH = –log[H⁺], the higher [H⁺] from nitric acid gives a lower pH, while the lower [H⁺] from methanoic acid gives a higher pH.
Examiner tips
- Show the ionisation equations for each acid. Explain ‘complete’ vs ‘partial’ ionisation. Link [H⁺] to pH using the formula. Use the correct units and symbols.
Common mistakes
- Confusing concentration units (mol dm⁻³ vs mol L⁻¹). Assuming both acids give the same [H⁺] because concentrations are equal. Forgetting to include the negative sign in the pH formula.
Mark scheme (5 marks)
- Strong acids fully/completely ionise (in solution)
- Weak acids only partially ionise (in solution)
- Nitric acid produces a higher concentration of hydrogen ions (H⁺) than methanoic acid at the same concentration
- Lower concentration of H⁺ ions (from methanoic acid) gives a higher pH
- pH is a measure of hydrogen ion concentration, so a higher [H⁺] gives a lower pH
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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