Find the oxidation number of (a) oxygen in H₂O₂; (b) oxygen in OF₂; (c) oxygen in KO₂; (d) hydrogen…

Step 1. (a). $2(+1) + 2x = 0$, so $x = -1$. It is a peroxide.

Step 2. (b). Fluorine is $-1$. $x + 2(-1) = 0$, so $x = +2$.

Step 3. (c). Potassium is $+1$. $(+1) + 2x = 0$, so $x = -\tfrac{1}{2} = -0.5$. It is a superoxide.

Step 4. (d). Calcium is $+2$. $(+2) + 2x = 0$, so $x = -1$. It is a metal hydride.

Step 5. (e). $2(+1) + 2x + 6(-2) + 2(-1) = 0$, so $2x = 12$ and $x = +6$.