Chemistry 9701 · AS & A Level · Equilibria

Equilibria — practice question

Chromium(III) hydroxide, $\text{Cr(OH)}_3$, has only a small solubility in water. At 298 K, its solubility product, $K_{sp}$, is $1.0 \times 10^{-33}$.
(a(i))[2]

Complete the $K_{sp}$ expression for $\text{Cr(OH)}_3$, and include the units.

(a(ii))[3]

At 298 K, Calculate the solubility of $\text{Cr(OH)}_3$ in pure water, in $\text{g dm}^{-3}$. Show your working.

(a(iii))[1]

Why is $\text{Cr(OH)}_3$ less soluble in $0.100\ \text{mol dm}^{-3}$ NaOH than it is in pure water? Explain this observation.

(b(i))[2]

At 298 K, Calculate the pH of $0.100\ \text{mol dm}^{-3}$ $\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$. Show your working.

(b(ii))[1]

At 298 K, Calculate the pH of $0.100\ \text{mol dm}^{-3}$ NaOH.

(b(iii))[2]

$5.00\ \text{cm}^3$ of $0.100\ \text{mol dm}^{-3}$ NaOH is added to $10.00\ \text{cm}^3$ of $0.100\ \text{mol dm}^{-3}$ $\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$. Calculate the pH of the resulting solution. Show your working.

(c)[2]

An aqueous solution of volume $80.0\ \text{cm}^3$ that contains $0.704\ \text{g}$ of $\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$ is shaken with $100\ \text{cm}^3$ benzene, $\text{C}_6\text{H}_6$. At equilibrium, $0.556\ \text{g}$ of $\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$ is present in the $100\ \text{cm}^3$ of $\text{C}_6\text{H}_6$. Calculate the partition coefficient, $K_{pc}$, of $\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$ between $\text{C}_6\text{H}_6$ and water. Show your working.

(iii)[2]

$5.00\ \text{cm}^3$ of $0.100\ \text{mol dm}^{-3}$ NaOH is added to $10.00\ \text{cm}^3$ of $0.100\ \text{mol dm}^{-3}$ $\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$. Calculate the pH of the resulting solution. Show your working.

Worked solution & mark scheme

This 15-mark question has a full step-by-step worked solution and mark scheme. One marking point: The expression is $K_{sp} = [\mathrm{Cr^{3+}}][\mathrm{OH^-}]^3$.

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