Hi!
1. Assume that these percentages are the mass.
2. For each of Fe and O, divide the mass by the Ar (relative atomic mass).
Fe: 72.4/55.8 = 1.297 moles
O: 27.6/16 = 1.725 moles
3. Work out which mole value is smallest, then divide the bigger mole value by the smaller one.
1.297 is smaller than 1.725, so your empirical formula is 1.1297/1.1297 = 1 atom of iron, and 1.725/1.297 = 1.33 atoms of oxygen.
4. Ensure your atom values are whole numbers.
1.33 is not a whole number, so multiply both 1 and 1.33 by 3.
5. Write out your empirical formula.
Fe3O4
Hope that helps!
