Understanding the relationship between the masses of atoms and the chemical formulas of compounds allows us to quantitatively describe the composition of substances.
In an earlier chapter, we described the development of the atomic mass unit, the concept of average atomic masses, and the use of chemical formulas to represent the elemental makeup of substances.
Sodium chloride is an ionic compound composed of sodium cations, Na Note that the average masses of neutral sodium and chlorine atoms were used in this computation, rather than the masses for sodium cations and chlorine anions.
This approach is perfectly acceptable when computing the formula mass of an ionic compound.
For example, if you have mixed 25g of table salt, or Na Cl, into 2 liters of water, you first need to determine how many moles are in 25g of Na Cl.