DP Chemistry (last assessment 2024)
Question 19N.2.sl.TZ0.a(i)
| Date | November 2019 | Marks available | [Maximum mark: 1] | Reference code | 19N.2.sl.TZ0.a(i) |
| Level | sl | Paper | 2 | Time zone | TZ0 |
| Command term | Determine | Question number | a(i) | Adapted from | N/A |
a(i).
[Maximum mark: 1]
19N.2.sl.TZ0.a(i)
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).
Determine the mole ratio of S2O32− to O2, using the balanced equations.
[1]
Markscheme
4 : 1 ✔
