A-Level Chemistry May/June 2024 Q6(a)(ii): The partition coefficient, Kpc, for a compound, X, between carbon disulfide, CS2, and w…

1 answer

• ▲0
  accepted ✓

  First, calculate the mass of X in the 40.0 cm³ of aqueous solution used for the extraction.

  Mass of X in 40.0 cm³ = $\frac{1.85 \text{ g}}{100.0 \text{ cm}^3} \times 40.0 \text{ cm}^3 = 0.74 \text{ g}$

  Let $y$ be the mass of X, in g, extracted into the CS₂ layer. The mass of X remaining in the aqueous layer is $(0.74 - y)$ g.

  The partition coefficient, $K_{pc}$, is given by: $K_{pc} = \frac{\text{Concentration of X in CS}_2}{\text{Concentration of X in water}} = \frac{\text{mass}_\text{CS₂} / \text{volume}_\text{CS₂}}{\text{mass}_\text{H₂O} / \text{volume}_\text{H₂O}}$

  Substitute the known values into the expression:

  $10.5 = \frac{y / 25.0}{(0.74 - y) / 40.0}$

  Now, rearrange and solve for $y$:

  $10.5 = \frac{y}{25.0} \times \frac{40.0}{0.74 - y}$

  $10.5 = \frac{1.6y}{0.74 - y}$

  $10.5 \times (0.74 - y) = 1.6y$

  $7.77 - 10.5y = 1.6y$

  $7.77 = 1.6y + 10.5y$

  $7.77 = 12.1y$

  $y = \frac{7.77}{12.1} = 0.64214...$

  Mass of X in CS₂ = $0.642$ g (to 3 s.f.)

  ---

  How the marks are awarded

  • M1 — Correctly setting up the partition coefficient expression. This requires first calculating the mass in the 40.0 cm³ sample (0.74 g) and then forming the ratio of concentrations: (mass in CS₂ / 25.0) divided by (mass in water / 40.0), and equating this to 10.5.
  • M2 — Correctly calculating the final answer of 0.642 g by successfully rearranging and solving the algebraic expression established for the M1 mark.

  Common mistakes

  • Using the total initial mass of 1.85 g in the Kpc calculation instead of first calculating the mass in the 40.0 cm³ aliquot (0.74 g).
  • Inverting the Kpc expression, i.e., placing the concentration in water in the numerator and the concentration in carbon disulfide in the denominator.
  • Incorrectly swapping the volumes, for example, dividing the mass in CS₂ by 40.0 cm³ and the mass in water by 25.0 cm³.
  • Making an algebraic error when rearranging the equation to solve for 'y', such as incorrectly expanding the brackets.

  Examiner tip: Always determine the actual mass of solute present in the specific volume being used for the extraction before setting up the partition coefficient equation.

  ---

  AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →

  @markscheme_answers

Your answer

Sign in to answer this question.