DP Chemistry (last assessment 2024)

Question 22M.2.sl.TZ1.e(iii)

e(iii).

[Maximum mark: 1]

22M.2.sl.TZ1.e(iii)

Ammonia, NH₃, is industrially important for the manufacture of fertilizers, explosives and plastics.

Ammonia is soluble in water and forms an alkaline solution:

NH₃(g) + H₂O (l) $⇌$ NH₄⁺(aq) + HO^–(aq)

Calculate the concentration of hydroxide ions in an ammonia solution with pH = 9.3. Use sections 1 and 2 of the data booklet.

[1]

Markscheme

[OH⁻] $⟨ ⟨ = \frac{K_{W}}{[H^{+}]} = \frac{10^{- 14}}{10^{- 9.3}} = 10^{- 4.7} ⟩ ⟩ = 2.0 \times 10^{- 5} ⟨ ⟨ mol {dm}^{- 3} ⟩ ⟩$ ✔

Examiners report

This question was very poorly answered with many candidates calculating the [H⁺] instead of [OH^-].

Syllabus sections