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Date	May 2019	Marks available	2	Reference code	19M.3.sl.TZ2.16
Level	SL	Paper	3	Time zone	TZ2
Command term	Outline	Question number	16	Adapted from	N/A

Question

Excess acid in the stomach can cause breakdown of the stomach lining.

Outline how ranitidine (Zantac) inhibits stomach acid production.

[1]

a(i).

Outline two advantages of taking ranitidine instead of an antacid which neutralizes excess acid.

[2]

a(ii).

Some antacids contain carbonates.

Determine the pH of a buffer solution which contains 0.160 mol dm⁻³ CO₃²⁻ and 0.200 mol dm⁻³ HCO₃⁻, using section 1 of the data booklet.

pK_a (HCO₃⁻) = 10.32

[1]

Markscheme

blocks/binds H2/histamine receptors «in cells of stomach lining»
OR
prevents histamine molecules binding to H2/histamine receptors «and triggering acid secretion» [✔]

a(i).

Any two of:
ranitidine can be effective in treating ulcers «but antacid is not» [✔]

ranitidine can prevent long-term damage «from overproduction of acid and antacid does not» [✔]

ranitidine has a long-term effect «and antacid has short-term effect only» [✔]

ranitidine does not affect ionic balance in body «and antacid does» [✔]

ranitidine does not produce bloating/flatulence [✔]

Note: Accept “ranitidine stops the over production of acid in the stomach while antacids neutralize the excess acid giving temporary relief”.

a(ii).

«pH = pK_a + log $\frac{{{\text{[}}{{\text{A}}^ - }]}}{{[{\text{HA]}}}}$ = 10.32 + log $\frac{{0.160}}{{0.200}}$ = 10.32 – 0.097»

«pH =»10.22 [✔]

Examiners report

This question was not very well answered even though it has been seen in previous scripts. Some students were not through enough in their answers to receive a mark while others confused the action of ranitidine which blocks H2 receptors with omeprazole which is a proton pump inhibitor.

a(i).

While most students were awarded at least one of the two marks possible for this question some of the descriptions were too vague or incomplete to receive a mark.

a(ii).

This was in general a well answered question. Most candidates who did not receive the mark inverted the concentration of the conjugate base/concentration of the acid in the calculation.

Syllabus sections

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

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