Question

What is the oxidation number of carbon in the ion (Co3)2-?

Answer 1

The carbon is fully oxidized. Therefore, it is `C, IV^+`.

Explanation:

The sum of the formal oxidation numbers in an a complex or compound ion must sum to the charge of the ion. Here the charge is `-2`. Oxygen typically has an oxidation number of `-II`, and it does here. So `3 xx -2 + C_(ON) = -2`. And what therefore is `C_(ON)`?

What is the oxidation number of carbon in oxalate ion, `C_2O_4^(2-)`?

Answer 2

The oxidation number of the carbon is +4.

Explanation:

Oxygen normally has an oxidation state of -2.

Oxygen = three ions each with a charge of -2 = 3 x -2 = -6
The overall charge of the polyatomic ion is -2

This means that the oxidation number of the carbon must be +4.

Total charge = +4 + -6 = -2

Here is a video which discusses this topic by discussing how to determine oxidation numbers of elements in the bicarbonate and dichromate ions.

Video from: Noel Pauller

Hope this helps!