Determine the empirical formula of an oxide of iron which has 69.9% iron and 30.1% dioxygen by mass.

% of iron by mass = 69.9 % [Given]

% of oxygen by mass = 30.1 % [Given]

Relative moles of iron in iron oxide:

Relative moles of oxygen in iron oxide:

Simplest molar ratio of iron to oxygen:

= 1.25: 1.88

= 1: 1.5

2: 3

∴ The empirical formula of the iron oxide is Fe₂O_3.