A student tests two solutions, A and B, both at the same concentration. Solution A is sodium hydroxide and solution B is ammonia solution. Both solutions have a pH above 7, but solution A has a higher pH than solution B. Explain why solution A has a higher pH than solution B, even though both solutions are at the same concentration.

WJEC A-Level Chemistry (Wales) — 3.9 Acid-base equilibria · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Sodium hydroxide is a strong base and therefore it completely dissociates in water to give a high concentration of OH⁻ ions.
Ammonia is a weak base; it only partially dissociates and an equilibrium between NH₃, NH₄⁺ and OH⁻ is established.
Because the OH⁻ concentration in the NaOH solution is higher than that in the ammonia solution, the NaOH solution has a higher pH even though both solutions are at the same concentration.

Examiner tips

  • State that NaOH is a strong base and fully dissociates. State that NH₃ is a weak base and only partially dissociates. Explain that the higher OH⁻ concentration gives the higher pH.

Common mistakes

  • Confusing the concentration of the base with the concentration of OH⁻. Forgetting that NH₃ is a weak base and only partially dissociates. Using the wrong order of explanation (e.g., saying NH₃ gives more OH⁻).

Mark scheme (5 marks)

  1. Sodium hydroxide is a strong base / alkali
  2. Sodium hydroxide completely / fully dissociates (ionises) in water
  3. Ammonia is a weak base / alkali
  4. Ammonia only partially dissociates (ionises) in water / an equilibrium is established
  5. Therefore solution A has a higher concentration of OH⁻ ions than solution B (at the same concentration), giving it a higher pH

Key terms in this question

pH

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