The relative formula mass of a substance is the sum of the relative atomic masses of the atoms present in the formula. In case of FeSO4 we can calculate as follows: Relative atomic mass of O=16, S=32, Fe=56 So, FeSO4= (1x56)+(1x32)+(4x16) = 152 Calculating Moles. Moles= mass / Molar Mass Molar Mass is equal to the relative atomic or formula mass of the element or substance respectively. In case of FeSO4 it is 152 and mass has been given as 380g. So, Moles= 380/152 = 2.5moles