CAIE A-Level Chemistry A2 25.2 Partition Coefficients Questions

Practise defining and calculating equilibrium distribution of a solute between two immiscible solvents.

Exam points

define Kpc as the ratio of equilibrium solute concentrations in the two stated solvents
calculate mass or moles remaining in each layer before dividing concentrations, not raw amounts
preserve the numerator/denominator order specified by the question and state both solvent phases

Covered syllabus points

25.2.1—Terms: partition coefficient, Kpc 25.2.2—And use a partition coefficient for a system 25.2.3—Factors affecting partition coefficient

[Maximum number: 4]

Procaine is used as an anaesthetic in medicine. It can be synthesised from methylbenzene in five steps as shown in Fig. 7.1.

Fig. 7.1

(a)

Explain what is meant by partition coefficient, $K_{\mathrm{pc}}$ .

[ 2 ]

(b)

The partition coefficient of procaine between octan-1-ol and water is 1.77.

Octan-1-ol and water are immiscible. A solution containing 0.500 g of procaine in $75.0 \mathrm{~cm}^{3}$ of water is shaken with $50.0 \mathrm{~cm}^{3}$ of octan-1-ol.

Calculate the mass of procaine that is extracted into the octan-1-ol. g

[ 2 ]

M1 $K_{\mathrm{pc}}=[\text { procaine }]_{\text {oct }} /[\text { procaine }]_{\text {water }}$ 1.77=(x / 50) /(0.5-x / 75)

M2 1.77=1.5 x / 0.5-x

0.885-1.77 x=1.5 x

x=0.271 gmin 2 sf

[Maximum number: 4]

Iodine is found naturally in compounds in many different oxidation states.

(a)

Iodide ions, $\mathrm{I}^{-}$ , react with acidified $\mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq})$ to form iodine, $\mathrm{I}_{2}$ , and water. This reaction mixture is shaken with cyclohexane, $\mathrm{C}_{6} \mathrm{H}_{12}$ , to extract the $\mathrm{I}_{2}$ . Cyclohexane is immiscible with water.

[ 4 ]

(i)

$15.0 \mathrm{~cm}^{3}$ of $\mathrm{C}_{6} \mathrm{H}_{12}$ is shaken with $20.0 \mathrm{~cm}^{3}$ of an aqueous solution containing $\mathrm{I}_{2}$ until no further change is seen.
It is found that 0.390 g of $\mathrm{I}_{2}$ is extracted into the $\mathrm{C}_{6} \mathrm{H}_{12}$ .
The partition coefficient of $\mathrm{I}_{2}$ between $\mathrm{C}_{6} \mathrm{H}_{12}$ and water, $K_{\mathrm{pc}}$ , is 93.8.
Calculate the mass of $\mathrm{I}_{2}$ that remains in the aqueous layer.
Show your working.
mass of $I_{2}$ in aqueous layer =

[ 2 ]

(ii)

Suggest how the value of $K_{p c}$ of $\mathrm{I}_{2}$ between hexan-2-one, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{3} \mathrm{COCH}_{3}$ , and water compares to the value given in (a)(ii). Explain your answer.

[ 2 ]

$\quad K_{\mathrm{pc}}$ would be lower

- hexan-2-one is more polar (than cyclohexane)

OR hexan-2-one is polar AND cyclohexane is non-polar

- $\quad \mathrm{I}_{2}$ is (therefore) less soluble in hexan-2-one

All three correct for two marks

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Covered syllabus points

Question 1

Question (a)

Question (b)

Question 2

Question (a)

Question (i)

Question (ii)