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Question

In the reaction: Mn2++S2O28SO24+MnO4 (acid medium), the number of moles of S2O28 required to oxidize 2 moles of Mn2+ is :

A
5
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B
6
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C
2
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D
none of these
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Solution

The correct option is A 5
The unbalanced chemical equation is :
Mn2++S2O28SO24+MnO4
Balance all atoms other than H and O,
Mn2++S2O282SO24+MnO4
The oxidation number of Mn changes from +2 to +7.
The increase in the oxidation number of Mn is 5.
The oxidation number of S changes from +7 to +6.
The decrease in the oxidation number of one S atom is 1.
Total decrease in the oxidation number of 2 S atoms is 2.
To balance the increase in the oxidation number with decrease in the oxidation number, multiply Mn2+ and MnO4 with 2 and multiply S2O28 and SO24 with 5.
Thus, the balanced reaction is as follows:
2Mn2++5S2O2810SO24+5MnO4
Hence, the oxidation of 2 moles of Mn2+ requires 5 moles of S2O28.

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