A buffer solution can be made by mixing propanoic acid (CH₃CH₂COOH) with sodium propanoate (CH₃CH₂COONa). Explain how this buffer solution resists a change in pH when a small amount of hydrochloric acid is added to it.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Propanoic acid is a weak acid with Ka = 1.34 × 10⁻⁵ mol dm⁻³. When sodium propanoate dissolves in water it fully dissociates to give propanoate ions (CH₃CH₂COO⁻) and sodium ions.
Model answer (5 marks)
A buffer contains a large amount of the conjugate base CH₃CH₂COO⁻. When HCl is added, the H⁺ ions react with the conjugate base:
CH₃CH₂COO⁻ + H⁺ → CH₃CH₂COOH
Thus the added H⁺ is consumed to form propanoic acid. Because propanoic acid is a weak acid it only partially dissociates, so the concentration of free H⁺ ions in solution does not rise significantly. Consequently the pH of the solution remains essentially unchanged, or changes only very slightly.
This explains why the buffer resists a change in pH when a small amount of HCl is added.
CH₃CH₂COO⁻ + H⁺ → CH₃CH₂COOH
Thus the added H⁺ is consumed to form propanoic acid. Because propanoic acid is a weak acid it only partially dissociates, so the concentration of free H⁺ ions in solution does not rise significantly. Consequently the pH of the solution remains essentially unchanged, or changes only very slightly.
This explains why the buffer resists a change in pH when a small amount of HCl is added.
Examiner tips
- Use the word ‘react’ to show the H⁺ consumption; mention the conjugate base and weak acid; explain partial dissociation; link to pH stability; keep answer concise and to the points.
Common mistakes
- Confusing the buffer components as a strong acid/base; failing to state that the added H⁺ is consumed; not mentioning partial dissociation of the weak acid; over‑expanding with irrelevant detail.
Mark scheme (5 marks)
- The buffer contains a high concentration of propanoate ions (CH₃CH₂COO⁻), which act as the conjugate base / reservoir of base
- The added H⁺ ions (from HCl) react with the propanoate ions
- This produces propanoic acid (CH₃CH₂COOH), converting the added H⁺ into a weak acid
- Because propanoic acid is a weak acid, it only partially dissociates, so the concentration of H⁺ ions in solution does not significantly increase
- Therefore the pH remains approximately constant / changes only very slightly
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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