Chemistry 9701/53 — May/June 2023

The partition coefficient, $K_{pc}$, shows the distribution of a solute between two immiscible solvents. $K_{pc}$ is determined by measuring the concentration of the solute in each solvent.

The organic solvent ethoxyethane, $\text{CH}_3\text{CH}_2\text{OCH}_2\text{CH}_3$, and water are immiscible. A student is asked to find $K_{pc}$ of butanedioic acid, $\text{HOOCCH}_2\text{CH}_2\text{COOH}$, between ethoxyethane and water.

The expression for $K_{pc}$ when butanedioic acid is in equilibrium between ethoxyethane and water is shown.

$$K_{pc} = \frac{[\text{HOOCCH}_2\text{CH}_2\text{COOH}(\text{ethoxyethane})]}{[\text{HOOCCH}_2\text{CH}_2\text{COOH}(\text{aq})]}$$

[density: ethoxyethane, $0.71\text{ g cm}^{-3}$; water, $1.00\text{ g cm}^{-3}$]

The student uses the following method to find the partition coefficient. A diagram of the apparatus is shown in Fig. 1.1.

step 1 Add $30.0\text{ cm}^3$ of distilled water to a separating funnel.

step 2 Weigh by difference $2.81\text{ g}$ of butanedioic acid into the separating funnel.

step 3 Stopper the separating funnel and shake it until the butanedioic acid has dissolved.

step 4 Remove the stopper and add $30.0\text{ cm}^3$ of ethoxyethane to the separating funnel.

step 5 Replace the stopper and shake the separating funnel gently.

step 6 Place the separating funnel into a clamp. Allow the liquids to settle so that the two layers can be seen.

step 7 Remove the stopper and open the separating funnel tap to allow the lower layer to run into a beaker labelled A. Run the upper layer into a beaker labelled B.

step 8 Transfer $10.0\text{ cm}^3$ of the aqueous layer into a conical flask. Titrate with $0.500\text{ mol dm}^{-3}$ $\text{NaOH(aq)}$. Use thymolphthalein as the indicator.

step 9 Take $10.0\text{ cm}^3$ of the ethoxyethane layer and add $10.0\text{ cm}^3$ of water to it. Titrate this mixture with $0.100\text{ mol dm}^{-3}$ $\text{NaOH(aq)}$. Use thymolphthalein as the indicator.