wikiHow Grams to Moles Calculator To convert grams to moles, you need to divide the number of grams by the molar mass of the substance. Here is the formula: Number of moles = Number of grams / Molar mass The molar mass is the mass of one mole of a substance and is expressed in grams per mole (g/mol). You can find the molar mass of an element or compound by adding up the atomic masses of all the atoms in the formula of the substance. For example, to convert 10 grams of water (H2O) to moles, you would use the formula as follows: Find the molar mass of water by adding up the atomic masses of hydrogen and oxygen: Molar mass of H2O = 2(1.008 g/mol) + 1(15.999 g/mol) = 18.015 g/mol Divide the number of grams by the molar mass: Number of moles of H2O = 10 g / 18.015 g/mol = 0.555 moles So 10 grams of water is equal to 0.555 moles of water. Page