DP Chemistry (last assessment 2024)

Question 22M.2.hl.TZ1.c(ii)

Date	May 2022	Marks available	[Maximum mark: 2]	Reference code	22M.2.hl.TZ1.c(ii)
Level	hl	Paper	2	Time zone	TZ1
Command term	Calculate	Question number	c(ii)	Adapted from	N/A

c(ii).

[Maximum mark: 2]

22M.2.hl.TZ1.c(ii)

Ammonia is soluble in water and forms an alkaline solution:

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

Calculate the concentration, in mol dm^–3, of ammonia molecules in the solution with pH = 9.3. Use section 21 of the data booklet.

[2]

Markscheme

$K_{b} = \frac{[N H_{4}^{+}] [O H^{-}]}{[N H_{3}]} / \frac{10^{- 4.7} \times 10^{- 4.7}}{[N H_{3}]} ⟨ ⟨ = 10^{- 4.75} ⟩ ⟩$ ✔

$[{NH}_{3}] = ⟨ ⟨ = \frac{10^{- 9.4}}{10^{- 4.75}} = 10^{- 4.65} ⟩ ⟩ = 2.24 \times 10^{- 5} « mol {dm}^{- 3} »$ ✔

Accept other methods of carrying out the calculation.

Award [2] for correct answer.

Examiners report

Mediocre performance. Since a mark was given for the K_b expression, that mark could also be scored for the Henderson Hasselbalch (HH) equation, provided it is specific to the equilibrium reaction. Unfortunately, there was poor understanding of the application of the equation in most cases. Students should be strongly encouraged to use the HH equation only when a buffer is involved. Appropriate K_a or K_b expressions should be used when buffer solutions are not involved.

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Additional higher level (AHL)

Additional higher level (AHL) » Topic 18: Acids and bases » 18.2 Calculations involving acids and bases

Additional higher level (AHL) » Topic 18: Acids and bases

Question 22M.2.hl.TZ1.c(ii)

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