DP Chemistry (last assessment 2024)

Question 19N.2.sl.TZ0.2a(ii)

Date	November 2019	Marks available	[Maximum mark: 2]	Reference code	19N.2.sl.TZ0.2a(ii)
Level	sl	Paper	2	Time zone	TZ0
Command term	Calculate	Question number	a(ii)	Adapted from	N/A

a(ii).

[Maximum mark: 2]

19N.2.sl.TZ0.2a(ii)

The biochemical oxygen demand of a water sample can be determined by the following series of reactions. The final step is titration of the sample with sodium thiosulfate solution, Na₂S₂O₃ (aq).

2Mn²⁺ (aq) + O₂ (aq) + 4OH⁻ (aq) → 2MnO₂ (s) + 2H₂O (l)

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

2S₂O₃²⁻ (aq) + I₂ (aq) → 2I⁻ (aq) + S₄O₆²⁻ (aq)

A student analysed two 300.0 cm³ samples of water taken from the school pond: one immediately (day 0), and the other after leaving it sealed in a dark cupboard for five days (day 5). The following results were obtained for the titration of the samples with 0.0100 mol dm⁻³ Na₂S₂O₃ (aq).

(a(ii))

Calculate the number of moles of oxygen in the day 0 sample.

[2]

Markscheme

${{\text{n}}_{{{\text{s}}_2}{{\text{o}}_3}^{2 - }}} = «0.0258{\text{ d}}{{\text{m}}^3} \times 0.010{\text{ mol d}}{{\text{m}}^{ - 3}} =» 2.58 \times {10^{ - 4}}{\text{«mol»}}$ ✔

« $\frac{{2.58 \times {{10}^{ - 4}}{\text{mol}}}}{4} =» 6.45 \times {10^{ - 5}}{\text{«mol»}}$ ✔

NOTE: Award [2] for correct final answer.

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Core

Core » Topic 1: Stoichiometric relationships » 1.2 The mole concept

Core » Topic 1: Stoichiometric relationships

Question 19N.2.sl.TZ0.2a(ii)

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