DP Chemistry (last assessment 2024)

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Question 19M.2.hl.TZ1.5d(iii)

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Date May 2019 Marks available [Maximum mark: 2] Reference code 19M.2.hl.TZ1.5d(iii)
Level hl Paper 2 Time zone TZ1
Command term Explain Question number d(iii) Adapted from N/A
d(iii).
[Maximum mark: 2]
19M.2.hl.TZ1.5d(iii)

Both vinegar (a dilute aqueous solution of ethanoic acid) and bleach are used as cleaning agents.

(d(iii))

Explain, using two equations, how an equimolar solution of ammonia and ammonium ions acts as a buffer solution when small amounts of acid or base are added.

[2]

Markscheme

NH3 (aq) + H+ (aq) → NH4 + (aq)    [✔]

NH4 + (aq) + OH− (aq) → NH3 (aq) + H2O(l)    [✔]

 

Note: Accept reaction arrows or equilibrium signs in both equations.

Award [1 max], based on two correct reverse equations but not clearly showing reacting with acid or base but rather dissociation.

Examiners report

Nearly 30 % of candidates did not attempt to answer this question about buffer equations. It was also poorly answered because equations were not used to explain buffer action or the dissociation equations for the base and acid were given rather than their reactions with H+ or OH- .

Syllabus sections