Question

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

Answer 1

Ferric oxide, `Fe_2O_3`.

Explanation:

What's the other name for this stuff?

The iron is assumed to be in a `+III` oxidation state, i.e. we treat it as `Fe^(3+)`. Oxygen generally assumes a `-II` 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: `3xx(-2) +2xx3=0`, as required.