The oxidation state of carbon in the oxalate ion (C₂O₄²⁻) is +3. Here's how to determine it:

* Oxygen's oxidation state: Oxygen typically has an oxidation state of -2, except in peroxides.

* Charge of the ion: The oxalate ion has a charge of -2.

* Setting up the equation: Let x be the oxidation state of carbon. We have two carbons and four oxygens:

2x + 4(-2) = -2

* Solving for x:

2x - 8 = -2

2x = 6

x = +3

Therefore, the oxidation state of carbon in the oxalate ion is +3.