Chemistry 9701 · AS & A Level · Equilibria

Equilibria — practice question

Ethoxyethane, $\text{C}_2\text{H}_5\text{OC}_2\text{H}_5$, is soluble in both water and octan-1-ol. The partition coefficient for ethoxyethane between water and octan-1-ol is expressed below, together with its numerical value. Water and octan-1-ol are immiscible. $K_{pc} = \frac{\text{concentration of } \text{C}_2\text{H}_5\text{OC}_2\text{H}_5 \text{ in octan-1-ol}}{\text{concentration of } \text{C}_2\text{H}_5\text{OC}_2\text{H}_5 \text{ in water}} = 6.760 \text{ at } 20^{\circ}\text{C}$.
(a)[2]

An experiment is carried out in which octan-1-ol at $20^{\circ}\text{C}$ is introduced into a solution of ethoxyethane in water at $20^{\circ}\text{C}$. The mixture is then analysed immediately and a value of $K_{pc}$ is determined. The working is correct; the result obtained is 5.625. Explain why the calculated value is below 6.760.

(b(i))[1]

A second experiment is carried out and $K_{pc}$ is found to be 6.760. In the octan-1-ol phase, the ethoxyethane concentration is $7.62\ \text{g dm}^{-3}$. Calculate the concentration, in $\text{g dm}^{-3}$, of ethoxyethane in the aqueous phase.

(b(ii))[3]

A sample of $100 \text{cm}^3$ of the octan-1-ol layer is collected and shaken with $100 \text{cm}^3$ of water. Calculate the greatest amount, in mol, of ethoxyethane that can be transferred into the water.

(c(i))[2]

An aqueous solution of lead(II) nitrate is combined with an aqueous solution of sodium iodide. A yellow precipitate of lead(II) iodide is produced and is filtered off, leaving solution $X$. The concentration of $\text{Pb}^{2+}$ in solution $X$ is $5.68 \times 10^{-3}\ \text{mol dm}^{-3}$. The concentration of $\text{I}^-$ in solution $X$ is $4.20 \times 10^{-4}\ \text{mol dm}^{-3}$. Use these values to calculate a value for the solubility product, $K_{sp}$, of lead(II) iodide. State the units of $K_{sp}$.

(c(ii))[2]

Potassium iodide dissolves readily in water. Describe and explain the observation if a few drops of saturated potassium iodide solution are introduced into a portion of solution $X$.

Worked solution & mark scheme

This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: equilibrium has not been reached / not at equilibrium

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