急--titration of Fe(NH4)2(SO4)2．xH2O and KMnO4

Question

State the functions of sulphuric acid.Guess what happens if aqueous sulphuric acid is not enough in this titration.

Update:

can you give more functions of sulphuric acid?

Answer 1

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Sulphuric acid is used to acidify the reaction mixture.

In a strongly acidic medium, MnO4-(aq) is reduced to colourless Mn2+(aq), while Fe2+(aq) is oxidized to yellow Fe3+(aq).

The end point is reached when the solution turns pale pink (or pale purple) due to a slight excess of MnO4-.

MnO4-(aq) + 8H+(aq) + 5Fe2+(aq) → Mn2+(aq) + 4H2O(l) + 5Fe3+(aq)

If the solution is not strongly acidic enough or even neutral, Fe2+(aq) is also oxidized to Fe3+(aq) by the MnO4. However, MnO4-(aq) is reduced to MnO2(s), which is a brown solid. In the presence of brown MnO2 solid, the end point colour (pale pink or pale purple) is difficult to determine.

MnO4-(aq) + 4H+(aq) + 3Fe2+(aq) → MnO2(s) + 2H2O(l) + 3Fe3+(aq)