DP Chemistry (last assessment 2024)

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Question 18M.2.hl.TZ1.e

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Date May 2018 Marks available [Maximum mark: 2] Reference code 18M.2.hl.TZ1.e
Level hl Paper 2 Time zone TZ1
Command term Explain, Formulate Question number e Adapted from N/A
e.
[Maximum mark: 2]
18M.2.hl.TZ1.e

Limescale, CaCO3(s), can be removed from water kettles by using vinegar, a dilute solution of ethanoic acid, CH3COOH(aq).

Explain, using appropriate equations, how a suitably concentrated solution formed by the partial neutralization of 2,2-dimethylpropanoic acid with sodium hydroxide acts as a buffer solution.

[2]

Markscheme

(CH3)3CCOOH(aq) + OH–(aq) → (CH3)3CCOO–(aq) + H2O(l)

OR

(CH3)3CCOOH(aq) + OH–(aq) ⇌  (CH3)3CCOO–(aq) + H2O(l) AND addition of alkali causes equilibrium to move to right

 

(CH3)3CCOO–(aq) + H+(aq) → (CH3)3CCOOH(aq)

OR

(CH3)3CCOO–(aq) + H+(aq) ⇌ (CH3)3CCOOH(aq) AND addition of acid causes equilibrium to move to right

 

Accept “HA” for the acid.

Award [1 max] for correct explanations of buffering with addition of acid AND base without equilibrium equations.

[2 marks]

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