Chemistry 9701 · AS & A Level · Equilibria

Equilibria — practice question

(a(i))[2]

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

(a(ii))[3]

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

(a(iii))[1]

$\text{Cr(OH)}_3$ has a lower solubility in $0.100\,\text{mol dm}^{-3}$ NaOH than in pure water. Explain this observation.

(b(i))[2]

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

(b(ii))[1]

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

(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 solution formed. Show your working.

(c)[2]

An aqueous solution of volume $80.0\,\text{cm}^3$ containing $0.704\,\text{g}$ of $\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$ is shaken with $100\,\text{cm}^3$ of 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.

Worked solution & mark scheme

This 13-mark question has a full step-by-step worked solution and mark scheme. One marking point: Expression: $K_{sp} = [\text{Cr}^{3+}][\text{OH}^-]^3$

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