The mass of a substance is different from the quantity of a substance not only in physical terms but also in terms of units. The mass of a substance is a basic physical quantity, refers to the amount of material contained in the object, its SI unit is kilogram (kg); the amount of matter is also a basic physical quantity, its Sl unit is the mole (mo1). Such as 1 mol O2, that is, the amount of oxygen is 1 mo1, its mass is 32 g.

Molar mass is a unit of material quantity of the substance has the mass of the unit is grams / mo or kilograms / mo (g / mo1 or kg / mo1) and so on. Molar mass in grams / moles as a unit, numerically equal to the relative molecular mass or relative atomic mass of substances. For a pure substance, its molar mass is fixed, while the mass of a substance changes with the amount of substance. For example, 1 mol of O2 has a mass of 32 g, and 2 mol of O2 has a mass of 64 g, but the molar mass of O2 does not change, it is still 32 g/mol