DP Chemistry (last assessment 2024)

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Question 19N.3.sl.TZ0.16

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Date November 2019 Marks available [Maximum mark: 4] Reference code 19N.3.sl.TZ0.16
Level sl Paper 3 Time zone TZ0
Command term Calculate, Formulate, Identify, Outline, State Question number 16 Adapted from N/A
16.
[Maximum mark: 4]
19N.3.sl.TZ0.16

Body fluids have different pH values.

(a)

Identify the compound responsible for the acidity of gastric juice, and state whether it is a strong or weak acid.

[1]

Markscheme

hydrochloric acid/HCl «(aq)» AND strong «acid» ✔

(b)

An antacid contains calcium carbonate and magnesium carbonate.

Write the equation for the reaction of magnesium carbonate with excess stomach acid.

[1]

Markscheme

MgCO3 (s) + 2HCl (aq) → MgCl2 (aq) + CO2 (g) + H2O (l) ✔

NOTE: Accept ionic equation.

(c)

Outline how ranitidine reduces stomach acidity.

[1]

Markscheme

blocks/binds to H2-histamine receptors «in cells of stomach lining»
OR
prevents histamine molecules binding to H2-histamine receptors «and triggering acid secretion»
OR
prevents parietal cells from releasing/producing acid ✔

NOTE: Do not accept “antihistamine” by itself.
Accept “H2-receptor antagonist/H2RA” OR “blocks/inhibits action of histamine”.
Accept “blocks receptors in parietal cells «from releasing/producing acid»”.
Do not accept “proton pump/ATPase inhibitor”.

(d)

Calculate the pH of a buffer solution which contains 0.20 mol dm−3 ethanoic acid and 0.50 mol dm−3 sodium ethanoate. Use section 1 of the data booklet.

pKa (ethanoic acid) = 4.76

[1]

Markscheme

«pKa = 4.76»
«pH = pKa + log [CH3COO−][CH3COOH]»
«pH = 4.76 + 0.40 =» 5.16 ✔

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