| Date | May 2014 | Marks available | 2 | Reference code | 14M.3.sl.TZ1.16 |
| Level | SL | Paper | 3 | Time zone | TZ1 |
| Command term | Determine | Question number | 16 | Adapted from | N/A |
Question
Dissolved oxygen is used up when organic matter is decomposed aerobically in water.
The Winkler method, which is based on redox reactions, can be used to determine the concentration of dissolved oxygen in water.
A \({\text{200 c}}{{\text{m}}^{\text{3}}}\) sample of water was taken from a river and analysed using this method.
The redox reactions are shown below.
Step 1 \({\text{2M}}{{\text{n}}^{2 + }}{\text{(aq)}} + {\text{4O}}{{\text{H}}^ - }{\text{(aq)}} + {{\text{O}}_2}{\text{(aq)}} \to {\text{2Mn}}{{\text{O}}_2}{\text{(s)}} + {\text{2}}{{\text{H}}_2}{\text{O(l)}}\)
Step 2 \({\text{Mn}}{{\text{O}}_2}{\text{(s)}} + {\text{2}}{{\text{I}}^ - }{\text{(aq)}} + {\text{4}}{{\text{H}}^ + }{\text{(aq)}} \to {\text{M}}{{\text{n}}^{2 + }}{\text{(aq)}} + {{\text{I}}_2}{\text{(aq)}} + {\text{2}}{{\text{H}}_2}{\text{O(l)}}\)
Step 3 \({\text{2}}{{\text{S}}_2}{\text{O}}_3^{2 - }{\text{(aq)}} + {{\text{I}}_2}{\text{(aq)}} \to {{\text{S}}_4}{\text{O}}_6^{2 - }{\text{(aq)}} + {\text{2}}{{\text{I}}^ - }{\text{(aq)}}\)
(i) \(1.50 \times {10^{ - 4}}{\text{ mol}}\) of \({{\text{I}}^ - }{\text{(aq)}}\) was formed in step 3. Determine the amount, in mol, of oxygen, \({{\text{O}}_{\text{2}}}{\text{(aq)}}\), dissolved in the water.
(ii) Determine the solubility, in \({\text{g}}\,{\text{d}}{{\text{m}}^{ - 3}}\), of the oxygen in the water.
Markscheme
(i) \(\left( {\frac{{1.50 \times {{10}^{ - 4}}}}{4} = } \right){\text{ }}3.75 \times {10^{ - 5}}{\text{ (mol)}}\);
(ii) \(\left( {\frac{{3.75 \times {{10}^{ - 5}} \times 1000 \times 32.00}}{{200}} = } \right){\text{ }}6.00 \times {10^{ - 3}}{\text{ }}({\text{g}}\,{\text{d}}{{\text{m}}^{ - 3}})\);
Examiners report
Candidates performed poorly on this question and were not able to gain the mark for part (a). Many did not apply the terms oxidation and reduction correctly and some did not distinguish between the two terms, only one of the two terms was explained. Some incorrectly stated hydrogen used in anaerobic respiration instead of oxygen being absent. Several also missed reference to organic material. Several candidates were able to perform the calculations, for part (b), correctly; some used incorrect value for \({{\text{I}}^ - }\) moles (2 instead of 4) in part (ii) and did not gain the mark.
