Chemistry 9701 · AS & A Level · Equilibria

Equilibria — practice question

The table gives several organic acids together with their $pK_a$ values.
(a(i))[1]

State how $pK_a$ is connected to acid strength.

(a(ii))[1]

State the formula linking $pK_a$ with the acidity constant $K_a$.

(a(iii))[2]

Give reasons why the $pK_a$ value for chloroethanoic acid is lower than the value for ethanoic acid.

(b(i))[1]

Use the zwitterionic structure of aminoethanoic acid (glycine) in aqueous solution to write an equation showing its dissociation to give $\text{H}^+(aq)$ ions.

(b(ii))[2]

Calculate the pH of a $0.100\,\text{mol dm}^{-3}$ solution of aminoethanoic acid.

(b(iii))[3]

A $10.0\,\text{cm}^3$ portion of $0.100\,\text{mol dm}^{-3}$ aminoethanoic acid (glycine) was titrated using $0.100\,\text{mol dm}^{-3}$ NaOH. Once $20.0\,\text{cm}^3$ of NaOH, in excess, had been added, the pH was measured as 12.5. With the axes below, sketch a graph to show how the pH varies during this titration.

Worked solution & mark scheme

This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: A smaller $pK_a$ indicates a stronger acid.

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