DP Chemistry (last assessment 2024)

Question 21N.2.sl.TZ0.c(iii)

c(iii).

[Maximum mark: 2]

21N.2.sl.TZ0.c(iii)

Biochemical oxygen demand (BOD) can be determined by the Winkler Method.

A 25.00 cm³ sample of water was treated according to the Winkler Method.

Step I: 2Mn²⁺(aq) + O₂(g) + 4OH⁻(aq) → 2MnO₂(s) + 2H₂O (l)

Step II: MnO₂(s) + 2I⁻(aq) + 4H⁺(aq) → Mn²⁺(aq) + I₂(aq) + 2H₂O (l)

Step III: 2S₂O₃²⁻(aq) + I₂(aq) → 2I⁻(aq) + S₄O₆²⁻(aq)

The iodine produced was titrated with 37.50 cm³ of 5.000 × 10⁻⁴mol dm⁻³ Na₂S₂O₃.

Calculate the concentration of dissolved oxygen, in mol dm⁻³, in the sample.

[2]

Markscheme

« $1.875 \times 10^{- 5} mol \times \frac{1}{4} =$ » 4.688 × 10⁻⁶«mol» ✔

« $\frac{4.688 \times 10^{- 6} mol}{\frac{25.00 {cm}^{3}}{1000}} =$ » 1.875 × 10⁻⁴«mol dm⁻³» ✔

Award [2] for correct final answer.

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