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MOLAR MASS the mass of a mole of any element or compound in grams The mass of one mole of carbon atoms HOW TO CALCULATE THE MOLAR MASS OF A COMPOUND: 1. You need to know how many atoms of each element are in a substance in order to calculate its molecular weight. For example, H2O has two atoms of hydrogen and one atom of oxygen. 2. You need to know the molar mass of each element in order to calculate the molar mass of the compound. You can find these values on your periodic table. 3. Multiply each element's molar mass by how many atoms are present in the formula, and then add the answers. To calculate the molar mass of H2O: Hydrogen: two atoms, molar mass = 1.01g/mol Oxygen: one atom, molar mass = 16.00 g/mol (2 x 1.01g) + (1 x 16.00g) = 18.02g/mol Here’s another example: Calculate the molar mass of Al2(SO4)3: two atoms of aluminum and the molar mass of Al is 26.98g/mol three atoms of sulfur and the molar mass of S is 32.07g/mol twelve atoms of oxygen and the molar mass of O is 16.00g/mol (2 x 26.98g/mol) + (3 x 32.07g/mol) + (12 x 16.00g/mol) = 342.17g/mol Calculate the molar mass of: 1) (NH4)2S 2) Fe2O3 3) KClO4 4) SF6 5) (NH4)2SO4