Question

When potassium permanganate is allowed to react with ferrous ammonium sulphate in acidic medium the equivalent weight of potassium permanganate is?

Answer 1

Answer:

Explanation: When Potassium Permanganate reacts with Mohr's Salt ie.FeSO4.(NH4)2SO4.6H2O, the equivalent weight of Mohr's Salt is actually equal to its own molecular weight

How?

Let's assume that Molecular Weight of Mohr's Salt is 'M'.

KMnO4 will oxidise Fe2+ ion to Fe3+ and will itself reduce to Mn2+

However, Fe2+ changes it's oxidation state by 1. Hence N-Factor or Valence Factor of Fe in Mohr's Salt is 1 and hence Equivalent Weight=Molecular Weight/N-Factor = M/1 = 1.

Here, most people get confused by the fact that the net change of electrons should be equal to 5. However, that has nothing to do with the change in equivalent weight of Mohr's Salt as the only component that oxidises is FeSO4. Ammonium Sulphate doesn't oxidise and so does water. Hence the only active component is Fe2+ and thus its N-factor is 1.

Instead of FeSO4, if someone asks you what's the n-factor of KMnO4, then it's answer is 5 and hence it's equivalent weight is one fifth of it's molecular weight.