When the substance is said to be an oxide of iron, that means the compound contains elements of Fe and O. Suppose you have 100 g of the sample.
Mass of Fe = 72.36 g
Mass of O = 100 - 72.36 = 27.64 g
The molar mass of Fe is 55.8 g/mol while the molar mass of O is 16 g/mol.
Moles of Fe = 72.36/55.8 = 1.29677
Moles of O = 27.64/16 = 1.7275
Now, divide both amounts by the lower number, which is 1.29677.
Fe: 1.29677/1.29677 = 1
O = 1.7275/1.29677 = 1.333
The answer must be whole numbers. So, let's try multiplying the least number to O that would make it a whole number. That would be 3.
Fe: 1*3 = 3
O: 1.333*3 = 4
Thus, the empirical formula is Fe₃O₄.