This question is about manganese compounds. Some data are given below.
Electrode reaction | / V | |
1 |
⇌ Mn |
+0.56 |
2 | + 2H2O + 2e− ⇌ MnO2 + 4OH− | +0.59 |
3 | Fe3+ + e− ⇌ Fe2+ | +0.77 |
4 | MnO2 + 4H+ + 2e− ⇌ Mn2+ + 2H2O | +1.23 |
5 |
+ 8H+ + 5e− ⇌ Mn2+ + 4H2O |
+1.51 |
6 | + 4H+ + 3e− ⇌ MnO2 + 2H2O | +1.70 |
7 | + 4H+ + 2e− ⇌ MnO2 + 2H2O | +2.26 |
State symbols are not required.
Steel is an alloy of iron and carbon. A group of students determined the iron content of a sample of steel wire by a titration method.
A known mass of the wire was dissolved in dilute sulfuric acid and the resulting solution made up to 250.0 cm3 with more dilute sulfuric acid and mixed thoroughly.
Fe + H2SO4 → FeSO4 + H2
25.0 cm3 samples of the resulting solution were titrated with 0.0195 mol dm−3 potassium manganate(VII) solution.
Using half-equations 3 and 5 from the table, calculate the percentage of iron in the steel wire. Give your answer to three significant figures.
A brown suspension formed during the titration.
Explain how, if at all, the titre value would be affected by this student’s error.
The uncertainties of the apparatus used in the experiment in (b) are shown.
Apparatus | Value measured | Uncertainty on each reading |
Percentage uncertainty on value measured / % |
Balance | 1.53 g | ±0.005 g | 0.65 |
Burette | 27.35 cm3 | ±0.05 cm3 | |
Pipette | 25.0 cm3 | ±0.06 cm3 | |
Volumetric flask | 250.0 cm3 | ±0.3 cm3 |
State whether or not this student has given their answer to an appropriate number of significant figures. Justify your answer in terms of the total percentage uncertainty of the experiment.
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