A student measures the pH of two solutions: a 0.1 mol/dm³ solution of sodium hydroxide and a 0.1 mol/dm³ solution of ammonia. The sodium hydroxide solution has a pH of 13, while the ammonia solution has a pH of 11. 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).
Sodium hydroxide is classified as a strong base. Ammonia is classified as a weak base.
Model answer (5 marks)
Sodium hydroxide is a strong base and dissociates completely in water:
NaOH → Na⁺ + OH⁻
Thus a 0.1 mol dm⁻³ NaOH solution contains 0.1 mol dm⁻³ OH⁻.
Ammonia is a weak base and only partially ionises:
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
At 0.1 mol dm⁻³ NH₃ the equilibrium lies to the left, so only a small fraction of NH₃ produces OH⁻.
Because the NaOH solution has a higher concentration of OH⁻ ions than the NH₃ solution, its pOH is lower and its pH is higher. Therefore the NaOH solution (pH 13) is more basic than the NH₃ solution (pH 11).
NaOH → Na⁺ + OH⁻
Thus a 0.1 mol dm⁻³ NaOH solution contains 0.1 mol dm⁻³ OH⁻.
Ammonia is a weak base and only partially ionises:
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
At 0.1 mol dm⁻³ NH₃ the equilibrium lies to the left, so only a small fraction of NH₃ produces OH⁻.
Because the NaOH solution has a higher concentration of OH⁻ ions than the NH₃ solution, its pOH is lower and its pH is higher. Therefore the NaOH solution (pH 13) is more basic than the NH₃ solution (pH 11).
Examiner tips
- Show the dissociation equations for both bases; mention ‘complete’ vs ‘partial’.
- Explain that higher [OH⁻] gives lower pOH and higher pH.
- Use the word ‘equilibrium lies to the left’ to justify fewer OH⁻ from NH₃.
Mark scheme (5 marks)
- Sodium hydroxide is fully/completely dissociated (in aqueous solution)
- Ammonia is only partially dissociated / partially ionised (in aqueous solution)
- This means the concentration of hydroxide ions (OH⁻) is higher in the sodium hydroxide solution than in the ammonia solution
- A higher concentration of OH⁻ ions results in a higher pH
- The equilibrium in the ammonia solution lies to the left / favours the undissociated form, so fewer OH⁻ ions are produced
Key terms in this question
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