Dicarboxylic acids ionise in successive stages.
$\text{HO}_2\text{C}(\text{CH}_2)_n\text{CO}_2\text{H} \rightleftharpoons \text{HO}_2\text{C}(\text{CH}_2)_n\text{CO}_2^- + \text{H}^+$ (stage 1)
$\rightleftharpoons \text{O}_2\text{C}(\text{CH}_2)_n\text{CO}_2^{2-} + 2\text{H}^+$ (stage 2)
For comparison, ethanoic acid, $\text{CH}_3\text{CO}_2\text{H}$, has a $pK_a$ of $4.76$.
(a(i))[1]
State the equation that links $pK_a$ with the acid dissociation constant $K_a$.
(a(ii))[3]
Using the table above, suggest why the $pK_a(1)$ values:
• are each lower than the $pK_a$ of ethanoic acid,
• increase as $n$ becomes larger.
(a(iii))[1]
Suggest why every $pK_a(2)$ value in the table above is larger than the $pK_a$ of ethanoic acid.
(b(i))[2]
Explain what the term buffer solution means.
(b(ii))[2]
Write two equations showing how monosodium butanedioate, $\text{HO}_2\text{CCH}_2\text{CH}_2\text{CO}_2\text{Na}$, functions as a buffer.
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