| Date | May 2009 | Marks available | 1 | Reference code | 09M.3.sl.TZ1.E2 |
| Level | SL | Paper | 3 | Time zone | TZ1 |
| Command term | State | Question number | E2 | Adapted from | N/A |
Question
Water purity is often assessed by reference to its oxygen content.
The Winkler method uses redox reactions to find the concentration of oxygen in water. \({\text{100 c}}{{\text{m}}^{\text{3}}}\) of water was taken from a river and analysed using this method. The reactions taking place are summarized below.
\[\begin{array}{*{20}{l}} {{\text{Step 1}}}&{{\text{2M}}{{\text{n}}^{2 + }}{\text{(aq)}} + {\text{4O}}{{\text{H}}^ - }{\text{(aq)}} + {{\text{O}}_2}{\text{(aq)}} \to {\text{2Mn}}{{\text{O}}_2}{\text{(s)}} + {\text{2}}{{\text{H}}_2}{\text{O(l)}}} \\ {{\text{Step 2}}}&{{\text{Mn}}{{\text{O}}_2}{\text{(s)}} + {\text{2}}{{\text{I}}^ - }{\text{(aq)}} + {\text{4}}{{\text{H}}^ + }{\text{(aq)}} \to {\text{M}}{{\text{n}}^{2 + }}{\text{(aq)}} + {{\text{I}}_2}{\text{(aq)}} + {\text{2}}{{\text{H}}_2}{\text{O(l)}}} \\ {{\text{Step 3}}}&{{\text{2}}{{\text{S}}_2}{\text{O}}_3^{2 - }{\text{(aq)}} + {{\text{I}}_2}{\text{(aq)}} \to {{\text{S}}_4}{\text{O}}_6^{2 - }{\text{(aq)}} + {\text{2}}{{\text{I}}^ - }{\text{(aq)}}} \end{array}\]
Outline the meaning of the term biochemical oxygen demand (BOD).
State what happened to the \({{\text{O}}_{\text{2}}}\) in step 1 in terms of electrons.
State the change in oxidation number for manganese in step 2.
0.0002 moles of \({{\text{I}}^ - }\) were formed in step 3. Calculate the amount, in moles, of oxygen, \({{\text{O}}_{\text{2}}}\), dissolved in water.
Markscheme
amount of oxygen needed to decompose organic matter;
in a specified time/five days / at a specified temp/ 20 °C;
Second mark can only be awarded if reasonable attempt made to define BOD.
gained electrons;
+4 to +2 / decrease by 2;
\(0.00005/5 \times {10^{ - 5}}\) (moles);
Examiners report
In part (a) the term biochemical oxygen demand (BOD) was not well known. Very few candidates could explain that it is related to the level of organic waste in the water measured at a specific temperature for a specific time period.
Many candidates understood that oxygen gained electrons.
Many candidates understood that the oxidation number of manganese dropped from +4 to +2.
Many candidates understood that oxygen gained electrons in (c) (i) and that the oxidation number of manganese dropped from +4 to +2 in (ii). However, they struggled to calculate the moles of dissolved oxygen.
