The mineral magnetite is an oxide of iron that is 72.36% iron by mass. what is its empirical formula?

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.