What is the oxidation number of the oxygen atom in the $O_2^{2-}$ ion?

$A$ compound contains atoms $X$,$Y$,and $Z$. The oxidation number of $X$ is $+2$,$Y$ is $+5$ and $Z$ is $-2$. Therefore,a possible formula of the compound is

What is the oxidation number of nitrogen in $N_3H$?

Write the oxidation number of oxygen in $KO_{3}$ and $Na_{2}O_{2}$.

The oxidation states of sulphur in $S_8$,$S_2F_2$,and $H_2S$ respectively,are:

What is the oxidation number of $As$ in $H_{3}AsO_{3}$?