Question

What is the oxidation number of Carbon in `Na_2C_2O_4`?

Answer 1

Each carbon in oxalate ion has an oxidation number of `+III`.

Explanation:

The sum of the oxidation numbers of the constituent atoms equals the charge on the ion. Oxygen normally has an oxidation number of `-II` in its compounds, and it certainly does here.

Thus `2xxC_"Oxidation number"+4xx(-2)=-2` (Of course, `-2` was the charge on the ion.) It does not take too much algebra to determine that `C_"Oxidation number"=+III`. Carbon here is almost fully oxidized.