Write the oxidation number of the carbon atom in the carboxyl group of acetic acid $(CH_3COOH)$.

(N/A) The structure of acetic acid is $CH_3-COOH$.
In $CH_3COOH$,the carbon atom in the methyl group $(CH_3)$ has an oxidation number of $-3$.
Let the oxidation number of the carbon atom in the carboxyl group $(-COOH)$ be $x$.
The sum of oxidation numbers of all atoms in the molecule is zero.
$(-3) + x + 2(-2) + (+1) = 0$
$-3 + x - 4 + 1 = 0$
$x - 6 = 0$
$x = +3$
Therefore,the oxidation number of the carbon atom in the carboxyl group is $+3$.