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Date	May 2018	Marks available	2	Reference code	18M.2.hl.TZ1.5
Level	HL	Paper	2	Time zone	TZ1
Command term	Determine	Question number	5	Adapted from	N/A

Question

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

Predict, giving a reason, a difference between the reactions of the same concentrations of hydrochloric acid and ethanoic acid with samples of calcium carbonate.

[2]

Dissolved carbon dioxide causes unpolluted rain to have a pH of approximately 5, but other dissolved gases can result in a much lower pH. State one environmental effect of acid rain.

[1]

Write an equation to show ammonia, NH₃, acting as a Brønsted–Lowry base and a different equation to show it acting as a Lewis base.

[2]

Determine the pH of 0.010 mol dm⁻³ 2,2-dimethylpropanoic acid solution.

K_a (2,2-dimethylpropanoic acid) = 9.333 × 10⁻⁶

[2]

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

slower rate with ethanoic acid

smaller temperature rise with ethanoic acid

[H⁺] lower

ethanoic acid is weak

ethanoic acid is partially dissociated

Accept experimental observations such as “slower bubbling” or “feels less warm”.

[2 marks]

Any one of:

corrosion of materials/metals/carbonate materials

destruction of plant/aquatic life

«indirect» effect on human health

Accept “lowering pH of oceans/lakes/waterways”.

[1 mark]

Brønsted–Lowry base:

NH₃ + H⁺ → NH₄⁺

Lewis base:

NH₃ + BF₃ → H₃NBF₃

Accept “AlCl₃as an example of Lewis acid”.

Accept other valid equations such as Cu²⁺ + 4NH₃→ [Cu(NH₃)₄]²⁺.

[2 marks]

[H⁺] « $= \sqrt {{{\text{K}}_{\text{a}}} \times \left[ {{{\text{C}}_5}{{\text{H}}_{10}}{{\text{O}}_2}} \right]} = \sqrt {9.333 \times {{10}^{ - 6}} \times 0.010} {\text{ }}$ » = 3.055 × 10^–4 «mol dm^–3»