DP Chemistry (last assessment 2024)

Question 17N.3.sl.TZ0.20

Date	November 2017	Marks available	[Maximum mark: 4]	Reference code	17N.3.sl.TZ0.20
Level	sl	Paper	3	Time zone	TZ0
Command term	Calculate, Explain	Question number	20	Adapted from	N/A

20.

[Maximum mark: 4]

17N.3.sl.TZ0.20

Excess acid in the stomach can cause discomfort and more serious health issues.

(a)

Explain how ranitidine (Zantac) reduces stomach acid production.

[2]

Markscheme

blocks/binds to H2/histamine receptors «in cells of stomach lining»

prevents histamine binding to H2/histamine receptors «and triggering acid secretion»

prevents parietal cells from releasing/producing acid

Accept "H2-receptor antagonist/H2RA" OR "blocks/inhibits action of histamine" for M1.

(b)

The pH is maintained in different fluids in the body by the use of buffers.

Calculate the pH of a buffer solution of 0.0200 mol dm^–3 carbonic acid, H₂CO₃, and 0.400 mol dm^–3 sodium hydrogen carbonate, NaHCO₃. The pK_a of carbonic acid is 6.35.

[2]

Markscheme

ALTERNATIVE 1

pH = «pK_a + log $\frac{{\left[ {{{\text{A}}^ - }} \right]}}{{\left[ {{\text{HA}}} \right]}}$ =» 6.35 + log ( $\frac{{0.400}}{{0.0200}}$ )

«pH =» 7.65

ALTERNATIVE 2

K_a = 4.5 x 10^–7

«K_a = 0.400 x $\frac{{\left[ {{{\text{H}}^ + }} \right]}}{{0.0200}}$ , [H⁺] =» 2.3 x 10^–8 «mol dm^–3»

«pH =» 7.64

Award [2] for correct final answer.

Do not accept “pH = 8”.

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

Options » D: Medicinal chemistry

Question 17N.3.sl.TZ0.20

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