In acetate, the oxidation number of carbon is +3, and the oxidation number of each oxygen atom is -2. There is also one hydrogen atom with an oxidation number of +1. Since the acetate ion has an overall charge of -1, the sum of the oxidation numbers of all the atoms must be -1.

Thus, let the oxidation number of acetate ion be \(x\). Then we can set up the equation as:

$$(+3) + 2(-2) + (+1) + x=-1$$

Solving for x, we get:

$$x=-2$$

Therefore, the oxidation number of acetate (CH3COO- ion) is -2.