Molar Mass of Sodium Chloride

Salt has no molecules, only a lattice, so 58.44 g/mol is strictly a formula mass: the mass of one mole of NaCl formula units. It is one of the few molar masses people use daily without noticing. Physiological saline at 0.9 % w/v is 9 g/L, which is 0.154 mol/L of NaCl and 0.308 osmol/L of particles once it dissociates — matched deliberately to blood plasma.

In water treatment the same figure sizes brine. A saturated NaCl brine at 20 °C holds about 26.4 % by mass, roughly 317 g/L or 5.4 mol/L, and an ion-exchange softener is dosed in kilograms of salt per cubic foot of resin against a theoretical exchange capacity expressed in equivalents. The uncertainty here, ±0.01 g/mol, comes almost entirely from chlorine, whose two stable isotopes vary enough between sources that IUPAC quotes the atomic weight only as 35.45.