If you want to know how many moles of a material you have, divide the mass of the material by its molar mass. The molar mass of a substance is the mass in grams of one mole of that substance. This mass is given by the atomic weight of the chemical unit that makes up that substance in atomic mass units (amu). For example, silver has an atomic weight of 107.8682 amu, so one mole of silver has a mass of 107.8682 grams.