# What is the chemical formula for iron(lll) oxide?

Ferric oxide, $F {e}_{2} {O}_{3}$.
The iron is assumed to be in a $+ I I I$ oxidation state, i.e. we treat it as $F {e}^{3 +}$. Oxygen generally assumes a $- I I$ oxidation state, i.e. ${O}^{2 -}$ and it does here.
We know that it is a neutral species, so we cross multiply the presumed charges to get the given formula: $3 \times \left(- 2\right) + 2 \times 3 = 0$, as required.