Chemistry 9701 · AS & A Level · Equilibria

Equilibria — practice question

(a) Chloric(I) acid, $\text{HClO}$, is a weak Brønsted-Lowry acid. If chloric(I) acid is mixed with aqueous sodium hydroxide, an acid-base reaction occurs, as shown: $\text{HClO} + \text{NaOH} \rightarrow \text{NaClO} + \text{H}_2\text{O}$.
(a(i))[1]

Identify the two conjugate acid-base pairs present in this reaction.

(a(ii))[2]

The value for the acid dissociation constant, $K_a$, of $\text{HClO(aq)}$ is $3.70 \times 10^{-8}$. Calculate the concentration, in $\text{mol dm}^{-3}$, of $\text{HClO(aq)}$ at a pH of 4.51.

(a(iii))[1]

When a solution of $\text{HClO(aq)}$ is heated, chloric(V) acid and a strong acid containing no oxygen are produced as the only products. Write an equation for this reaction.

(b(i))[1]

Define a buffer solution in words.

(b(ii))[1]

Suggest a substance that could be added to aqueous ethanoic acid to make a buffer solution. Explain your answer.

(c)[2]

Write two equations showing how $\text{H}_2\text{PO}_4^-$ ions can behave as a buffer.

(d(i))[3]

At $25\,^{\circ}\text{C}$, the solubility of calcium phosphate, $\text{Ca}_3(\text{PO}_4)_2$, is $1.14 \times 10^{-7}\,\text{mol dm}^{-3}$. The solubility product expression, $K_{sp}$, for $\text{Ca}_3(\text{PO}_4)_2$ is given as $K_{sp} = [\text{Ca}^{2+}]^3[\text{PO}_4^{3-}]^2$. Calculate $K_{sp}$ for $\text{Ca}_3(\text{PO}_4)_2$. Include units.

(d(ii))[1]

A small amount of solid sodium phosphate is added to a saturated solution of $\text{Ca}_3(\text{PO}_4)_2$. Predict the effect, if any, on the solubility of $\text{Ca}_3(\text{PO}_4)_2$. Explain your answer.

Worked solution & mark scheme

This 12-mark question has a full step-by-step worked solution and mark scheme. One marking point: Conjugate acid-base pairs correctly identified

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