DP Chemistry (last assessment 2024)

Question 22M.2.hl.TZ1.4c(i)

c(i).

[Maximum mark: 1]

22M.2.hl.TZ1.4c(i)

Ammonia is soluble in water and forms an alkaline solution:

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

(c(i))

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

Concentration of [OH^˗] was asked for but some calculated [H₃O⁺] instead. On the whole, question was done well.

Syllabus sections