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How many moles of KMnO4 are required to oxidise exactly one mole of sulphite ion (SO3 2-) in acidic medium?

1. 2/5
2. 3/5
3. 4/5
4. 1

Correct answer: 2/5

Solution

In acidic medium each MnO4- accepts 5 electrons while each SO3 2- gives 2 electrons on becoming SO4 2-, so 1 mole of sulphite needs 2/5 mole of KMnO4.

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