Question 19N.2.sl.TZ0.b(ii)
| Date | November 2019 | Marks available | [Maximum mark: 1] | Reference code | 19N.2.sl.TZ0.b(ii) |
| Level | sl | Paper | 2 | Time zone | TZ0 |
| Command term | Suggest | Question number | b(ii) | Adapted from | N/A |
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).
Suggest a modification to the procedure that would make the results more reliable.
[1]
repetition / take several samples «and average» ✔
