The molar mass of magnesium sulfate, which has a chemical formula of MgSO4, is 120.36 grams per mole. For any compound, the molar mass is calculated by adding together the atomic weight of each atom in the compound.
Each magnesium atom has an atomic weight of 24.3 grams per mole. The sulfur atom in the magnesium sulfate molecule carries an atomic weight of 32.06 grams per mole. Each individual atom of oxygen has an atomic weight of 15.99 grams per mole; the four atoms of oxygen in each magnesium sulfate molecule have a total weight of 63.96 grams per mole. The atomic weight of an element is typically listed on the periodic table.