Total mass = 2.75g Mass % of Mg = 1.0g x 100/2.75g = 36.36 % Mass % of O = 1.75g x 100/2.75g = 63.64 % Mol of Mg = 36.36/24 = 1.515 Mol of O = 63.64/16 = 3.977 Ratio of Mol of Mg and O in the substance = (1.515 : 3.977) x 2 = 3 : 8 The empirical formula of substance is Mg3O8