Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). It is calculated by adding the atomic masses of all the atoms in a molecule, which can be found on the periodic table. For example, the molar mass of water (H2O) is approximately 18.02 g/mol, derived from two hydrogen (H) atoms and one oxygen (O) atom. Understanding molar mass is essential in chemistry for converting between grams and moles, which helps in stoichiometric calculations. It allows chemists to determine how much of a substance is needed for a reaction or how much product can be formed from given reactants.