Chemistry 9701 · AS & A Level · Equilibria

Equilibria — practice question

Dicarboxylic acids dissociate stepwise. $\text{HO}_2\text{C(CH}_2)_n\text{CO}_2\text{H} \rightleftharpoons \text{HO}_2\text{C(CH}_2)_n\text{CO}_2^- + \text{H}^+$ (stage 1) $\text{HO}_2\text{C(CH}_2)_n\text{CO}_2^- \rightleftharpoons {}^-\text{O}_2\text{C(CH}_2)_n\text{CO}_2^- + 2\text{H}^+$ (stage 2)
(a(i))[1]

State the mathematical link between $pK_a$ and the acid dissociation constant $K_a$.

(a(ii))[3]

Using the table above, suggest why the $pK_a(1)$ values are all below the $pK_a$ of ethanoic acid, and why they increase as $n$ increases.

(a(iii))[1]

Using the table above, suggest why every $pK_a(2)$ value is greater than the $pK_a$ of ethanoic acid.

(b(i))[2]

Explain the meaning of the term buffer solution.

(b(ii))[2]

Write two equations to show how monosodium butanedioate, $\text{HO}_2\text{CCH}_2\text{CH}_2\text{CO}_2\text{Na}$, functions as a buffer.

Worked solution & mark scheme

This 9-mark question has a full step-by-step worked solution and mark scheme. One marking point: $pK_a = -\log K_a$

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