Groundwater from a borehole near a disused iron mine is suspected to be contaminated with iron(II) and iron(III) ions.
A student determines the concentration of iron(II) ions in a sample of the groundwater by titration with potassium manganate(VII).
25.0 cm325.0\text{ cm}^325.0 cm3 portions of groundwater are acidified with dilute sulfuric acid. Each portion is titrated with 0.00200 mol dm−30.00200\text{ mol dm}^{-3}0.00200 mol dm−3 potassium manganate(VII) until a permanent colour change is observed.
MnO4−(aq)+8H+(aq)+5Fe2+(aq)→Mn2+(aq)+4H2O(l)+5Fe3+(aq)\text{MnO}_4^-(\text{aq}) + 8\text{H}^+(\text{aq}) + 5\text{Fe}^{2+}(\text{aq}) \rightarrow \text{Mn}^{2+}(\text{aq}) + 4\text{H}_2\text{O}(\text{l}) + 5\text{Fe}^{3+}(\text{aq})MnO4−(aq)+8H+(aq)+5Fe2+(aq)→Mn2+(aq)+4H2O(l)+5Fe3+(aq)
State the colour change seen at the end point of the titration. from .................................... to ....................................
The student's titration results are shown in the table below. The trial titre has been omitted.
| 1 | 2 | 3 | |
|---|---|---|---|
| Final volume / cm3\text{cm}^3cm3 | 14.85 | 30.00 | 44.95 |
| Initial volume / cm3\text{cm}^3cm3 | 0.00 | 14.85 | 30.00 |
| Titre volume / cm3\text{cm}^3cm3 |
Complete the table above and calculate the mean titre that the student should use to determine the concentration of iron(II) ions in the groundwater.
mean titre = .................................... cm3\text{cm}^3cm3
Determine the concentration, in mol dm−3\text{mol dm}^{-3}mol dm−3, of iron(II) ions in the groundwater. Present your answer to three significant figures.
concentration = .................................... mol dm−3\text{mol dm}^{-3}mol dm−3
The student modifies the experiment to determine the combined concentration of iron(II) and iron(III) ions in the groundwater.
The student's method is shown below.
The table below shows information about three redox systems.
| Redox system | Half-equation | EθE^\thetaEθ / V\text{V}V |
|---|---|---|
| 1 | Zn2+(aq)+2e−⇌Zn(s)\text{Zn}^{2+}(\text{aq}) + 2\text{e}^- \rightleftharpoons \text{Zn}(\text{s})Zn2+(aq)+2e−⇌Zn(s) | −0.76-0.76−0.76 |
| 2 | Fe3+(aq)+e−⇌Fe2+(aq)\text{Fe}^{3+}(\text{aq}) + \text{e}^- \rightleftharpoons \text{Fe}^{2+}(\text{aq})Fe3+(aq)+e−⇌Fe2+(aq) | +0.77+0.77+0.77 |
| 3 | MnO4−(aq)+8H+(aq)+5e−⇌Mn2+(aq)+4H2O(l)\text{MnO}_4^-(\text{aq}) + 8\text{H}^+(\text{aq}) + 5\text{e}^- \rightleftharpoons \text{Mn}^{2+}(\text{aq}) + 4\text{H}_2\text{O}(\text{l})MnO4−(aq)+8H+(aq)+5e−⇌Mn2+(aq)+4H2O(l) | +1.51+1.51+1.51 |
Use the information in the table above to explain the reasons for Step 1 and Step 2.
Reason(s) for Step 1: ....................................
Reason(s) for Step 2: ....................................