Rinse liquor from an industrial steel-pickling bath is suspected to contain both iron(II) and iron(III) ions.
A chemist determines the concentration of iron(II) ions in a sample of the rinse liquor by titration with potassium manganate(VII).
20.0 cm320.0\text{ cm}^320.0 cm3 portions of the rinse liquor are acidified with dilute sulfuric acid. Each portion is titrated with 0.00250 mol dm−30.00250\text{ mol dm}^{-3}0.00250 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 chemist's titration results are shown in the table below. The trial titre has been omitted.
| 1 | 2 | 3 | |
|---|---|---|---|
| Final volume / cm3\text{cm}^3cm3 | 15.35 | 31.10 | 46.55 |
| Initial volume / cm3\text{cm}^3cm3 | 0.00 | 15.35 | 31.10 |
| Titre volume / cm3\text{cm}^3cm3 |
Complete the table above and calculate the mean titre that the chemist should use to determine the concentration of iron(II) ions in the rinse liquor.
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 rinse liquor. Present your answer to three significant figures.
concentration = .................................... mol dm−3\text{mol dm}^{-3}mol dm−3
The chemist modifies the experiment to determine the combined concentration of iron(II) and iron(III) ions in the rinse liquor.
The chemist'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: ....................................