Molar mass is a physical property defined as the mass of a given substance divided by the amount of substance. The base SI unit for molar mass is kg/mol. However, for historical reasons, molar masses are almost always expressed in g/mol. The molar mass of a substance is the total weight of that substance (in kg or g) for one mole of that substance. That is, the weight of a substance for 6.022140857E23 molecules (Avogadro constant) of that substance.