Chemistry 9701 · AS & A Level · Equilibria

Equilibria — practice question

Disodium phosphate, $(\text{Na}^+)_{2}(\text{HPO}_4^{2-})$, undergoes reaction with an acid to produce monosodium phosphate, $\text{Na}^+(\text{H}_2\text{PO}_4^{-})$.
(a(i))[1]

Using the species formulae in this reaction, identify the ions that make up a conjugate acid-base pair. Show which one is the conjugate acid and which one is the conjugate base.

(a(ii))[2]

What is meant by buffer solution?

(a(iii))[2]

Write two equations to demonstrate how a mixture of $(\text{Na}^+)_{2}(\text{HPO}_4^{2-})$ and $\text{Na}^+(\text{H}_2\text{PO}_4^{-})$ can function as a buffer solution.

(a(iv))[1]

Name one inorganic ion that acts as a buffer in blood.

(b(i))[2]

Compound E is the hydroxide of a Group 2 element. Compound E is a strong alkali. $2.63\,\text{g}$ of E is dissolved in water to make $250\,\text{cm}^3$ of solution F. Solution F has a pH of $13.09$ at $298\,\text{K}$. Show that the concentration of hydroxide ions in solution F is $0.123\,\text{mol dm}^{-3}$.

(b(ii))[1]

Explain why the concentration of compound E in solution F is $0.0615\,\text{mol dm}^{-3}$.

(b(iii))[1]

Use the concentration given in (ii) to identify compound E.

(c)[3]

Compound E is far more soluble than magnesium hydroxide. A saturated solution of magnesium hydroxide in water has a concentration of $1.40 \times 10^{-4}\,\text{mol dm}^{-3}$ at $298\,\text{K}$. Calculate the solubility product, $K_{sp}$, of magnesium hydroxide. Include units.

(d)[3]

Explain why compound E is much more soluble than magnesium hydroxide.

Worked solution & mark scheme

This 16-mark question has a full step-by-step worked solution and mark scheme. One marking point: The conjugate acid is $\mathrm{H_2PO_4^-}$ and the conjugate base is $\mathrm{HPO_4^{2-}}$

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