Question

what is the formula for iron(iii) oxide? ○ feo ○ fe₂o ○ feo₃ ○ fe₂o₃

Explanation:

Step1: Determine ion charges

Iron(III) means $\text{Fe}^{3+}$, oxide is $\text{O}^{2-}$.

Step2: Balance charges

To balance $3+$ and $2-$, find least common multiple of 3 and 2, which is 6. So we need 2 $\text{Fe}^{3+}$ (total $6+$) and 3 $\text{O}^{2-}$ (total $6-$).

Step3: Write formula

Combine ions: $\text{Fe}_2\text{O}_3$.

Answer:

$\boldsymbol{\text{Fe}_2\text{O}_3}$ (the last option, assuming the last option is $\text{Fe}_2\text{O}_3$ as per typical iron(III) oxide formula)