Acidic solution?

Question

Determine the number of water molecules necessary to balance the reduction half reaction of MnO4 ---> MnO2 that occurs in acidic solution.

Answer 1

In order to balance out the Oxygens, you would need to add 2 molecules of water to the product side. (MnO2 side).

4H+ + MnO4 ---> MnO2 + 2H2O + 4e-

I hope this helps.

Answer 2

To balance these you first find how many waters are required to balance to oxygens. So in this case you need two water molecules on the right. Since it is in acidic solution you balance the amount of added hydrogens from the water by adding four protons to the left.