Osmolarity is dependent upon the number of impermeant molecules in a solution, not on the identity of the molecules. For example, a 1M solution of a nonionizing substance such as glucose is a 1 Osmolar solution; a 1M solution of NaCl = 2 Osm; and a 1M solution of Na2SO4 =3 Osm. So in our example, the osmolarity of the 0.9% NaCl solution is 0.15M * 2 = 0.3 Osm.

Osmolarity is calulated in 2 steps:

Ex. for 0.3% NaCl

Molarity of NaCl = (0.3/100ml) / (molecular weight of NaCl)

Osmolarity = 2 * Molarity of NaCl