A-Level Chemistry October/November 2024 Q4(d)(ii): Calculate the concentration of the solution of Na2C2O4. [Na2C2O4] = moldm-3

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  Step 1: Calculate the moles of dichromate(VI) ions used.

  Amount of $Cr_2O_7^{2-} = \text{concentration} \times \text{volume}$

  Amount of $Cr_2O_7^{2-} = 0.0500 \text{ mol dm}^{-3} \times \frac{16.20}{1000} \text{ dm}^3$

  Amount of $Cr_2O_7^{2-} = 8.10 \times 10^{-4} \text{ mol}$

  Step 2: Use the molar ratio to find the moles of oxalate ions.

  The balanced redox equation shows a molar ratio of $Cr_2O_7^{2-} : C_2O_4^{2-}$ is $1:3$.

  Amount of $C_2O_4^{2-} = 3 \times \text{Amount of } Cr_2O_7^{2-}$

  Amount of $C_2O_4^{2-} = 3 \times 8.10 \times 10^{-4} \text{ mol}$

  Amount of $C_2O_4^{2-} = 2.43 \times 10^{-3} \text{ mol}$

  Step 3: Calculate the concentration of the $Na_2C_2O_4$ solution.

  This amount of $C_2O_4^{2-}$ was present in the $25.0 \text{ cm}^3$ sample of the solution.

  Concentration of $Na_2C_2O_4 = \frac{\text{amount}}{\text{volume}}$

  $[Na_2C_2O_4] = \frac{2.43 \times 10^{-3} \text{ mol}}{25.0 / 1000 \text{ dm}^3}$

  $[Na_2C_2O_4] = 0.0972 \text{ mol dm}^{-3}$ (3 s.f.)

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  How the marks are awarded

  • M1 — Correctly calculating the amount, in moles, of Cr₂O₇²⁻ ions by multiplying the given concentration (0.0500 mol dm⁻³) by the volume in dm³ (16.20/1000).
  • A1 — Correctly using the 1:3 molar ratio to find the moles of C₂O₄²⁻ (2.43 x 10⁻³ mol) and then correctly calculating the final concentration by dividing these moles by the volume of the oxalate solution (0.025 dm³) to get 0.0972 mol dm⁻³. This mark allows for an error carried forward (ECF) from the M1 calculation.

  Common mistakes

  • Using an incorrect molar ratio, such as 1:1 or 2:3, instead of the correct 1:3 ratio between dichromate(VI) and oxalate ions.
  • Forgetting to divide the volume in cm³ by 1000 to convert it to dm³, leading to an answer that is incorrect by a factor of 1000.
  • Inverting the molar ratio, for example by dividing the moles of dichromate(VI) by 3 instead of multiplying.
  • Giving the final answer to an inappropriate number of significant figures (e.g., 2 s.f. as 0.097) when the input data supports 3 s.f.

  Examiner tip: Master titration calculations by breaking them down into three distinct steps: calculate moles of the known substance, use the molar ratio to find moles of the unknown, and then calculate the final required value such as concentration or mass.

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