Explain why ammonia solution is classified as a weak base and describe how this affects the pH of a 0.1 mol/dm³ ammonia solution compared to a 0.1 mol/dm³ sodium hydroxide solution.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Ammonia is a weak base because it only partially ionises in water:
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
The equilibrium lies far to the left, so only a small fraction of NH₃ produces OH⁻.
Sodium hydroxide, in contrast, is a strong base and dissociates completely:
NaOH → Na⁺ + OH⁻
At the same concentration (0.1 mol dm⁻³) NaOH gives a much higher [OH⁻] than NH₃. A higher [OH⁻] corresponds to a higher pH, so the 0.1 mol dm⁻³ NH₃ solution has a lower pH than the 0.1 mol dm⁻³ NaOH solution.
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
The equilibrium lies far to the left, so only a small fraction of NH₃ produces OH⁻.
Sodium hydroxide, in contrast, is a strong base and dissociates completely:
NaOH → Na⁺ + OH⁻
At the same concentration (0.1 mol dm⁻³) NaOH gives a much higher [OH⁻] than NH₃. A higher [OH⁻] corresponds to a higher pH, so the 0.1 mol dm⁻³ NH₃ solution has a lower pH than the 0.1 mol dm⁻³ NaOH solution.
Examiner tips
- State that NH₃ is a weak base and give the equilibrium expression.
- Explain that NaOH is a strong base and fully dissociates.
- Show that higher [OH⁻] gives higher pH and conclude the pH comparison.
Common mistakes
- Confusing NH₃ with NH₄OH or writing the reaction as NH₃ + OH⁻ → NH₄⁺.
- Treating NH₃ as a strong base or claiming it gives the same [OH⁻] as NaOH.
- Ignoring the equilibrium direction or not linking [OH⁻] to pH.
Mark scheme (5 marks)
- A weak base is only partially ionised / dissociated in aqueous solution
- Ammonia reacts with water to produce OH⁻ ions / hydroxide ions, but the equilibrium lies to the left
- Sodium hydroxide is a strong base / fully ionised in solution, producing a higher concentration of OH⁻ ions than ammonia at the same concentration
- A higher concentration of OH⁻ ions means a higher pH
- Therefore the ammonia solution has a lower pH than the sodium hydroxide solution (of the same concentration)
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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