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A2 · Practice questions · Partition coefficients

Share it between layers.

Six original Cambridge-style questions on partition coefficients and solvent extraction.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Concentrations, not masses.

01
Recall
[2 marks]

Define the partition coefficient, Kpc, of a solute X between an organic solvent and water.

  • The ratio of the concentration of X in the organic solvent to its concentration in water, [X]organic ÷ [X]aqueous. ✓
  • Measured at equilibrium at a stated temperature, with the two solvents immiscible. ✓
02
Calculation
[2 marks]

After shaking, the hexane layer contains 0.0500 mol dm−3 of a solute and the water layer contains 6.00 × 10−4 mol dm−3. Calculate Kpc (hexane / water) and comment on the polarity of the solute.

  • Kpc = 0.0500 ÷ 6.00 × 10−4 = 83.3. ✓
  • It is much greater than 1, so the solute prefers hexane and is likely to be non-polar. ✓
03
Calculation
[3 marks]

2.00 g of Z is dissolved in 50.0 cm3 of water and shaken with 50.0 cm3 of an organic solvent. Kpc (organic / water) = 5.00. Calculate the mass of Z extracted.

  • Let x g be extracted: (x ÷ 50.0) ÷ ((2.00 − x) ÷ 50.0) = 5.00. ✓
  • x = 5.00(2.00 − x), so 6.00x = 10.0. ✓
  • x = 1.67 g extracted (0.333 g remains in the water). ✓
04
Calculation
[3 marks]

The same 2.00 g of Z in 50.0 cm3 of water (Kpc = 5.00) is instead extracted with two 25.0 cm3 portions. Calculate the total mass extracted and compare with the previous answer.

  • First portion: (x ÷ 25.0) ÷ ((2.00 − x) ÷ 50.0) = 5.00, so 2x = 5.00(2.00 − x), x = 1.43 g. ✓
  • Second portion, from 0.571 g left: 2y = 5.00(0.571 − y), y = 0.408 g. ✓
  • Total 1.84 g, more than 1.67 g with a single 50.0 cm3 portion. ✓
05
Explain
[2 marks]

Predict whether Kpc (hexane / water) is greater or less than 1 for (a) iodine and (b) ammonia. Explain.

  • (a) Greater than 1: I2 is non-polar and its id-id forces match those of hexane. ✓
  • (b) Less than 1: NH3 is polar and forms hydrogen bonds with water. ✓
06
Explain
[2 marks]

Explain why Kpc stays the same when more solute is added to the two-layer mixture at constant temperature.

  • Partition is a dynamic equilibrium, X(aq) ⇌ X(organic). ✓
  • Adding solute raises both concentrations in the same proportion, so the ratio, like any equilibrium constant, only changes with temperature. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.