Two drugs are ranitidine (Zantac) and omeprazole (Prilosec). Outline how they function to reduce stomach acidity.

0.500 g of solid anhydrous sodium carbonate, Na₂CO₃(s), is dissolved in 75.0 cm³ of 0.100 mol\(\,\)dm⁻³ sodium hydrogen carbonate solution, NaHCO₃(aq). Assume the volume does not change when the salt dissolves.

HCO₃⁻(aq) \( \rightleftharpoons \) CO₃²⁻(aq) + H⁺(aq)             pK_a = 10.35.

Calculate the pH of the buffer solution.

Ranitidine:
Blocks/binds H2-histamine receptors «in cells of stomach lining»
OR
prevents histamine molecules binding to H2-histamine receptors «and triggering acid secretion»

Omeprazole:
inhibits enzyme/gastric proton pump which secretes H⁺ ions «into gastric juice»

Accept “H2 receptor antagonist” for M1.

[2 marks]

[Na₂CO₃] = «\(\frac{{0.500{\text{ g}}}}{{105.99{\text{ g}}\,{\text{mo}}{{\text{l}}^{ - 1}} \times 0.075{\text{ d}}{{\text{m}}^3}}}\)=» 0.0629 «mol\(\,\)dm⁻³»

«pH = pK_a + log\(\frac{{[{\text{conj base]}}}}{{[{\text{conj acid}}]}}\)»

«pH = 10.35 – 0.201 =» 10.15

Alternative method involving K_a may be used to deduce pH in M2.

Award [2] for correct final answer.

-2 marks]