Molar mass is the mass of one mole of a substance. It is expressed in grams per mole (g/mol). The molar mass of an element is the sum of the atomic masses of all the atoms in the element's formula. For example, the molar mass of carbon (C) is 12.01 g/mol, which is the sum of the atomic masses of one carbon atom (12.01 amu).

The molar mass of a compound is the sum of the molar masses of all the atoms in the compound's formula. For example, the molar mass of water (H2O) is 18.02 g/mol, which is the sum of the molar masses of two hydrogen atoms (2 x 1.01 g/mol) and one oxygen atom (16.00 g/mol).

One mole of anything contains 6.022 x 10^23 particles of that substance. This number is known as Avogadro's number.