Molar mass is the mass of a substance that has 6.022 x 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 amount that has 6.022 x 1023 molecules. It can be found by adding the molar masses of all of its elements. So sodium hydroxide's (NaOH) molar mass can be found by adding the molar mass of sodium, oxygen, and hydrogen.