Calculate the pH of the buffer from the following data and section 1 of the data booklet.

pK_a(CO₂) = 6.34

[HCO₃⁻(aq)] = 1.40 × 10⁻² mol\(\,\)dm⁻³

[CO₂(aq)] = 1.25 × 10⁻³ mol\(\,\)dm⁻³

Explain the effect of a large amount of aspirin on the pH of blood.

«pH = pK_a + log₁₀ \(\left( {\frac{{\left[ {{\text{HC}}{{\text{O}}_3}^ - } \right]}}{{\left[ {{\text{C}}{{\text{O}}_2}} \right]}}} \right) = 6.34 + {\log _{10\;}}(11.2) = 6.34 + 1.05\)» = 7.39

[1 mark]

H⁺ from aspirin reacts with HCO₃⁻ to form CO₂ and H₂O
OR
H⁺(aq) + HCO₃⁻(aq) \( \rightleftharpoons \) CO₂(aq) + H₂O(l)
OR
reverse reaction favoured «to use up some of the H⁺ added»

pH decreases

No mark for “stating aspirin is a weak acid that dissociates partially to produce H⁺” without reference to reaction with HCO₃⁻ or to the equation.

Reversible arrows not required for the mark.

Do not accept “small pH change when small amount of H⁺ is added”.

[2 marks]