wikiHow Molar Mass Calculator Molar mass is calculated by adding up the atomic masses of all the atoms in a molecule, compound, or element. The atomic mass of an element is given in the periodic table and is usually expressed in atomic mass units (amu) or grams per mole (g/mol). To calculate the molar mass of a compound, you need to multiply the atomic mass of each element by its subscript in the chemical formula and then add up all the products. For example, the molar mass of water (H2O) can be calculated as follows: Molar mass of H2O = 2 x atomic mass of hydrogen + atomic mass of oxygen = 2 x 1.008 g/mol + 15.999 g/mol = 18.015 g/mol So the molar mass of water is 18.015 g/mol. Similarly, the molar mass of a larger compound such as glucose (C6H12O6) can be calculated by adding up the molar masses of all the atoms in the compound as follows: Molar mass of C6H12O6 = 6 x atomic mass of carbon + 12 x atomic mass of hydrogen + 6 x atomic mass of oxygen = 6 x 12.011 g/mol + 12 x 1.008 g/mol + 6 x 15.999 g/mol = 180.156 g/mol So the molar mass of glucose is 180.156 g/mol. Page