Propanoic acid (CH₃CH₂COOH) is a weak acid. Nitric acid (HNO₃) is a strong acid. Both acids can be prepared as solutions with the same concentration. Explain why, at the same concentration, a solution of nitric acid has a lower pH than a solution of propanoic acid.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Propanoic acid is a weak acid used as a food preservative. Nitric acid is a strong acid used in the manufacture of fertilisers. Both acids release hydrogen ions in aqueous solution.
Model answer (5 marks)
Nitric acid is a strong acid and therefore fully dissociates in water, giving one H⁺ (or H₃O⁺) per molecule.
Propanoic acid is a weak acid and only partially dissociates, so only a small fraction of the molecules release H⁺.
At the same concentration, the nitric‑acid solution therefore contains a higher concentration of H⁺ ions.
Because pH = –log[H⁺], a higher [H⁺] gives a lower pH.
The dissociation of propanoic acid is reversible, forming an equilibrium; not all molecules release H⁺, so its [H⁺] is lower than that of nitric acid.
Propanoic acid is a weak acid and only partially dissociates, so only a small fraction of the molecules release H⁺.
At the same concentration, the nitric‑acid solution therefore contains a higher concentration of H⁺ ions.
Because pH = –log[H⁺], a higher [H⁺] gives a lower pH.
The dissociation of propanoic acid is reversible, forming an equilibrium; not all molecules release H⁺, so its [H⁺] is lower than that of nitric acid.
Examiner tips
- State that nitric acid is a strong acid and fully dissociates. State that propanoic acid is weak and only partially dissociates. Explain that more H⁺ gives a lower pH. Use the pH definition to link [H⁺] to pH.
Common mistakes
- Confusing the terms ‘strong’ and ‘weak’ or saying both acids fully dissociate. Failing to mention the reversible equilibrium for the weak acid. Using the wrong sign when relating [H⁺] to pH (e.g. saying higher [H⁺] gives higher pH).
Mark scheme (5 marks)
- Nitric acid is a strong acid and fully/completely dissociates (ionises) in aqueous solution
- Propanoic acid is a weak acid and only partially/incompletely dissociates (ionises) in aqueous solution
- At the same concentration, nitric acid produces a greater concentration of hydrogen ions (H⁺/H₃O⁺) than propanoic acid
- A higher concentration of hydrogen ions corresponds to a lower pH
- Propanoic acid establishes a reversible equilibrium/the dissociation is reversible, so not all molecules release H⁺ ions
Key terms in this question
strong acid · weak acid · pH
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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