A-Level Chemistry May/June 2025 Q3(c): 80.0 cm³ of an aqueous solution containing 0.704g of CH3CH2CH2COOH is shaken with 100 c…

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  Working

  Step 1: Calculate the mass of butanoic acid in the aqueous layer at equilibrium.

  This is the initial mass minus the mass that moved into the benzene layer.

  Mass in water = Initial mass - Mass in benzene Mass in water = $0.704\ \text{g} - 0.556\ \text{g} = 0.148\ \text{g}$

  Step 2: Calculate the concentration of butanoic acid in each solvent.

  Concentration in benzene ($C_6H_6$): $[ ext{CH}_3 ext{CH}_2 ext{CH}_2 ext{COOH}]_{(\text{organic})} = \frac{\text{mass}}{\text{volume}} = \frac{0.556\ \text{g}}{100\ \text{cm}^3} = 0.00556\ \text{g cm}^{-3}$

  Concentration in water ($H_2O$): $[ ext{CH}_3 ext{CH}_2 ext{CH}_2 ext{COOH}]_{(\text{aqueous})} = \frac{\text{mass}}{\text{volume}} = \frac{0.148\ \text{g}}{80.0\ \text{cm}^3} = 0.00185\ \text{g cm}^{-3}$

  Step 3: Calculate the partition coefficient, Kpc.

  The partition coefficient is the ratio of the concentrations in the two immiscible solvents.

  $K_{\text{pc}} = \frac{[\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}]_{(\text{C}_6\text{H}_6)}}{[\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}]_{(\text{H}_2\text{O})}} = \frac{0.00556}{0.00185}$

  $K_{\text{pc}} = 3.0054...$

  Final Answer

  $K_{\text{pc}} = 3.01$ (to 3 significant figures)

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  How the marks are awarded

  • M1 — The first mark is awarded for correctly calculating the mass of butanoic acid remaining in the aqueous layer at equilibrium, which is 0.148 g.
  • M2 — The second mark is awarded for correctly calculating the final value of Kpc as 3.01 (or its inverse, 0.333) by first finding the concentrations in each solvent and then dividing them.

  Common mistakes

  • Calculating Kpc using the masses of the solute instead of the concentrations (e.g., 0.556 / 0.148), which ignores the different volumes of the solvents.
  • Forgetting to calculate the equilibrium mass in water and incorrectly using the initial mass (0.704 g) to find the aqueous concentration.
  • Mixing up the volumes when calculating concentrations, for example, by dividing the mass in benzene by the volume of water.
  • Making a simple subtraction error when finding the mass remaining in water, which leads to an incorrect final answer.

  Examiner tip: Always remember that the partition coefficient is a ratio of equilibrium concentrations, so you must divide the mass of the solute in each solvent by the volume of that specific solvent before calculating the final ratio.

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