Question 21N.2.sl.TZ0.6
| Date | November 2021 | Marks available | [Maximum mark: 8] | Reference code | 21N.2.sl.TZ0.6 |
| Level | sl | Paper | 2 | Time zone | TZ0 |
| Command term | Calculate, Deduce, Determine, Outline | Question number | 6 | Adapted from | N/A |
Biochemical oxygen demand (BOD) can be determined by the Winkler Method.
Outline what is measured by BOD.
[1]
«amount of» oxygen used to decompose the organic matter in water ✔
A student dissolved 0.1240 ± 0.0001 g of Na2S2O3 to make 1000.0 ± 0.4 cm3 of solution to use in the Winkler Method.
Determine the percentage uncertainty in the molar concentration.
[2]
«» 0.08 «%»
OR
«» 0.04 «%» ✔
«0.08 % + 0.04 % =» 0.12/0.1 «%» ✔
Award [2] for correct final answer.
Accept fractional uncertainties for M1, i.e., 0.0008 OR 0.0004.
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 × 10−4 mol dm−3 Na2S2O3.
Calculate the amount, in moles of Na2S2O3 used in the titration.
[1]
«× 5.000 × 10−4 mol dm−3 =» 1.875 × 10−5 «mol» ✔
Deduce the mole ratio of O2 consumed in step I to S2O32− used in step III.
[1]
1:4 ✔
Accept “4 mol S2O32– :1 mol O2“, but not just 4:1.
Calculate the concentration of dissolved oxygen, in mol dm−3, in the sample.
[2]
«» 4.688 × 10−6 «mol» ✔
«» 1.875 × 10−4 «mol dm−3» ✔
Award [2] for correct final answer.
The three steps of the Winkler Method are redox reactions.
Deduce the reduction half-equation for step II.
[1]
MnO2 (s) + 2e− + 4H+ (aq) → Mn2+ (aq) + 2H2O (l) ✔