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Date	November 2018	Marks available	2	Reference code	18N.3.sl.TZ0.14
Level	SL	Paper	3	Time zone	TZ0
Command term	Determine	Question number	14	Adapted from	N/A

Question

Buffer systems control pH in the body.

Determine the pH of a buffer solution that is 0.0100 mol dm⁻³ sodium hydrogen carbonate and 0.0200 mol dm⁻³ sodium carbonate, using section 1 of the data booklet.

K_a (hydrogen carbonate ion) = 4.8 × 10⁻¹¹

[2]

State the equation for the reaction of calcium carbonate, the active ingredient in some antacids, with stomach acid.

[1]

Suggest a technique for measuring the percentage mass of calcium carbonate in this type of antacid tablet.

[1]

Markscheme

ALTERNATIVE 1:

Using: pH = pK_a + log $\left( {\frac{{\left[ {{A^ - }} \right]}}{{\left[ {HA} \right]}}} \right)$

pK_a = 10.32 ✔

pH = «10.32 + log $\left( {\frac{{0.0200}}{{0.0100}}} \right)$ =» 10.62 ✔

ALTERNATIVE 2:

[H⁺] «= K_a × $\left( {\frac{{0.0100}}{{0.0200}}} \right)$ » = 2.4 × 10⁻¹¹ ✔

pH = 10.62 ✔

Award [2] for correct final answer.

Accept answers for M2 between 10.6 and 10.7.

Award [1 max] for pH = 10.02.

CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)

CaCO₃ (s) + 2H⁺ (aq) → Ca²⁺ (aq) + H₂O (l) + CO₂ (g) ✔

«back» titration

thermal decomposition

atomic absorption/AA ✔

Accept “gravimetric analysis”.

Do not accept description of a technique without proper term given for the technique.

Examiners report

[N/A]

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Options » D: Medicinal chemistry » D.4 pH regulation of the stomach

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