Help with redox question

Question

F‎‎‎‎‎‎ill in ‎‎the miss‎‎ing informa‎‎tion to balan‎‎ce the eq‎‎uation:

Hint: Mn‎‎O₂ is on‎‎e of the pr‎‎oducts, so w‎‎hat med‎‎ium sh‎‎ould y‎‎ou us‎‎e to balan‎ce? 

_H₂C‎‎‎‎‎O + _‎‎H₂‎‎‎‎O + _Mn‎‎O₄^- ⟶_H₂‎‎CO₂ + _Mn‎‎‎O₂ + __‎‎‎_

Answer 1

The hint is not much of a hint since MnO₂ is basically amphoteric, acting as both an acid or a base. It is a transition metal oxide, and the oxidation state of Mn is 4+. For transition metal oxides, the lowest oxidation state is generally basic, but 4+ is kind of in the middle. So, I'm not sure what the hint is meant to provide.

Here, I've balanced the redox equation in both acidic and basic medium. I think you have to be told whether to use acid or base, and you can't necessarily predict which is better. At least I'm not aware of how to choose. From the solutions, it appears they want you to balance it in basic medium since that gives you H₂O on the reactant side.

3H₂CO + 2MnO₄^- + 2H⁺ ==> 3H₂CO₂ + 2MnO₂ + H₂O ... in acid

3H₂CO + 2MnO₄^- + H₂O ==> 3H₂CO₂ + 2MnO₂ + 2OH^- ... in base