It is worth stressing that these osmolality readings are not just measuring the amount of Na+ and Cl– ions in the water but all soluble ions and compounds. For instance, sea water is known to contain many other ions including sulphate (SO4)2-, magnesium (Mg2+), calcium (Ca2+) and potassium (K+) ions,
Osmolality is a general measure of the particle concentration in a solute. It is not dependant on the nature of molecules but just their number. Therefore, a two molar solution of a non-dissociating molecule has the same osmolality as a one molar solution of a fully dissociating salt composed of two ions. The osmolality of a solution is the same, even when molecules vary in shape as well as in size. Therefore, all solutions containing the same number of osmotically active particles — regardless of their chemical properties — exhibit the same osmolality.