Question

What is the formula unit for a compound made from Fe3+ and oxygen?

Answer 1

Answer : The correct option is, $Fe_2O_3$

Explanation :

Formula unit of a compound is defined as the minimum number ratio of ions represented in the compound.

The valency of iron is (+3) and the valency of oxygen is (-2). By the criss-cross method the $Fe^{+3}$ cation combine with the $O^{2-}$ anion, it will give $Fe_2O_3$.

$Fe^{+3}+O^{2-}\rightarrow Fe_2O_3$

The formula by criss-cross method is shown below.

Answer 2

Fe2O3 because O has a -2 charge and Fe and +3, you want the charges to equal each other so if you have 2 Fe with +3 charge the charge becomes 6 if you have 3 O with -2 charge the charge becomes 6.