Question

Determine the molecular formula of an oxide of iron in which the mass per cent of iron and oxygen are $69.9$ and $30.1$ respectively.

Answer 1

The iron oxide has 69.9% iron and 30.1% dioxygen by mass.
Thus, 100 g of iron oxide contains 69.9 g iron and 30.1 g dioxygen.
The number of moles of iron present in 100 g of iron oxide are $\frac{69.9}{55.8}=1.25$.
The number of moles of dioxygen present in 100 g of iron oxide are $\frac{30.1}{32}=0.94$.
The ratio of the number of oxygen atoms to the number of carbon atoms present in one formula unit of iron oxide is $\frac{2\times 0.94}{1.25}=1.5:1=3:2$.
Hence, the formula of the iron oxide is $F{e}_2{O}_3$.