formula weight

Table salt does not exist as little NaCl molecules; it is a vast lattice of sodium and chloride ions. So how do you weigh 'one unit' of it? You add up the masses written in its formula — and that sum is the formula weight.

Formula weight is the sum of the atomic weights of all atoms shown in a chemical formula, expressed in atomic mass units (or grams per mole). It applies cleanly to ionic compounds and any substance that lacks discrete molecules, where the word 'molecular weight' would be misleading.

In day-to-day calculations, formula weight, molecular weight, and molar mass are computed exactly the same way and usually quoted as one number. The distinction is conceptual: 'formula weight' is the careful term for things, like salts, that have a formula but not true molecules.