Molar mass is the mass of a substance that has 6.022 × 10²³ atoms (Avogadro's number). The mass number of an element is equal to the molar mass of the element. The molar mass of a chemical compound is the mass when the compound has 6.022 × 10²³ molecules. Molar mass of chlorine is 35.45 due to consideration of average molar mass by including mass of its all isotopes.

It can be found by adding the atomic masses of all of its elements. The molar mass of sodium hydroxide (NaOH) can be found by adding the molar mass of sodium, oxygen, and hydrogen, which would be 39.997 g/mol.