Question

In what ratio will the iron iii ion and the oxide ion form an ionic compound

Answer 1

The oxidation state of iron may be 2,3 or 6 and the most common is 3 & this is our case, Fe^3+ can react with Oxygen (O2), which have fixed oxidation state (2), to form Fe2O3.
Two ions of iron and 3 ions of Oxygen with 2:3 ratio.