Molar mass is the mass of one mole of a substance, typically measured in grams per mole (g/mol). It represents the total mass of all the atoms in a molecule, calculated by adding the atomic masses of each element present in the compound. For example, the molar mass of water (H2O) is approximately 18 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 during chemical reactions. It allows chemists to determine how much of a substance is needed or produced in a reaction, facilitating accurate measurements and predictions.