67.2 g of iron was found to react with 28.8 g of oxygen. What is the empirical formula of iron oxide?

The first step is to calculate how many moles of iron and oxygen react together. Remember that moles = mass (g)/Ar (relative atomic mass). Given that the Ar of iron is 56 and the Ar of oxygen is 16, the moles of each are 67.2g/56 and 28.8g/16. So 1.2 moles of iron react with 1.8 moles of oxygen.The next step is to find out the proportion of moles of iron and moles of oxygen in the empirical formula. This is done by dividing each quantity in the compound by the lowest number of moles available. Since the lowest number of moles comes from iron (1.2), we divide each quantity of moles by 1.2. So we get proportions of (1.2/1.2) iron and (1.8/1.2) oxygen in the compound, which gives 1 mol of iron for every 1.5 mols of oxygen. To get the lowest whole number of moles of each element in the compound, we multiply both values by 2, giving 2 mols of iron for every 3 mols of oxygen in the formula. So the empirical formula of iron oxide is Fe₂O₃!