The half-cell reactions given below are relevant to the questions that follow.
Cl2 + 2e– → 2Cl– Eθ= +1.36 V
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Fe3+ + e– → Fe2+ Eθ = +0.77 V
MnO4– + 8H+ + 5e– → Mn2+ + 4H2O Eθ = +1.51 V
SO42– + 4H+ + 2e– → SO2 + 2H2O Eθ = +0.17 V
In order to standardise a solution of KMnO4, a student weighed out 5.56 g of FeSO4.7H2O, dissolved it in sulfuric acid and then made it up to a total volume of 250 cm3 with distilled water. She then took 25.0 cm3 portions of this solution, and added 10 cm3 of 2.00 mol dm–3 sulfuric acid to each. She then titrated these solutions against the potassium manganate(VII) solution. The average titre was 21.2 cm3.
a. Using the electrode potentials, explain why she used sulfuric acid and not hydrochloric acid in her titrations.
b. What was the concentration of the iron(II) sulfate solution?
c. i. Write the full ionic equation for the reaction between the manganate(VII) solution and the iron(II) sulfate.
ii. How does the student know that she has reached the end-point for the reaction?
d. What is the concentration of the manganate(VII) solution?
e. If the student passed sulfur dioxide gas through 25.0 cm3 of the manganate(VII) solution, what volume of gas would be required to completely decolorise the manganate(VII)? (1 mol of gas occupies 24.0 dm3 at room temperature and pressure)