# Oxygen almost always has an oxidation number of -2. Therefore in the formula for the mineral wüstite, FeO, what must the oxidation number for iron be? In the formula for hematite, Fe_2O_3, what must the oxidation number for iron be?

Actually oxygen usually has an oxidation number of $0$; certainly the dioxygen you and I breathe right now has this oxidation number.
In its oxides, an oxidation number of $- I I$ is typical. And since, for neutral compounds, the sum of the oxidation numbers is $0$. For $\text{ferrous oxide}$ we have $F {e}^{+ I I} {O}^{- I I}$; and for $\text{ferric oxide}$, $F {e}_{2}^{+ I I I} {O}_{3}^{- I I}$,
The $\text{ic/ous}$ ending generally specifies the higher oxidation state versus the lower. As an example stannous chloride, and titanous chloride are $S n C {l}_{2}$ and $T i C {l}_{2}$ respectively. Stannic chloride, and titanic chloride have the formulae, $S n C {l}_{4}$, and $T i C {l}_{4}$, and are distillable liquids.