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. Given that the molar mass of the oxide is 159.69 g mol^–1.

Answer 1

Mass percent of iron (Fe) = 69.9% (Given)

Mass percent of oxygen (O) = 30.1% (Given)

Number of moles of iron present in the oxide

= 1.25

Number of moles of oxygen present in the oxide

= 1.88

Ratio of iron to oxygen in the oxide,

= 1 : 1.5

= 2 : 3

The empirical formula of the oxide is Fe₂O_3.

Empirical formula mass of Fe₂O₃ = [2(55.85) + 3(16.00)] g

Molar mass of Fe₂O₃ = 159.69 g

Molecular formula of a compound is obtained by multiplying the empirical formula with n.

Thus, the empirical formula of the given oxide is Fe₂O₃ and n is 1.

Hence, the molecular formula of the oxide is Fe₂O_3.