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What is the oxidation number of carbon in the ion (Co3)2-?
The oxidation number of the carbon is +4.
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 of in the bicarbonate and dichromate ions. Video from: Noel Pauller
Hope this helps!