# Is osmolality preferred over osmolarity?

Jul 23, 2014

Yes, it is.

Colligative properties , such as freezing point depression, osmotic pressure, and vapour pressure, are independent of the kind and mass of particles of the solute, while depending only on the number of such particles in a unitary volume of solution (molarity) or mass of solvent (molality).

A colligative property is directly proportional to the concentration of all particles of all the various solutes in ideal solutions. Thus, the measure of a colligative property is not only a direct measure of molar or molal concentration, but also a collective measure of all the moles/molecules of solute.

Only very dilute solutions behave ideally. At these low concentrations, osmotic pressure is the most sensible and reliable measurable property. For this reason the "osmotic" adjective is used to refer to the total osmoles of all solutes, even though they have been calculated (most often) from freezing point depression measurements of solutions.

So we can understand the process leading to osmolality or osmolarity.
1. For a given complex solution (e.g. blood plasma, milk, serum, etc.) a colligative-collective property is measured.
2. From the experimental measure, a molality (moles per kilogram of solvent) or molarity (moles per liter of solution) are calculated assuming that the solution is ideal.
3. The calculated value is called osmolality or osmolarity.

Given that solute moles and solvent mass are not affected by pressure and temperature, the molality is unaffected too by these parameters. Thus, it is the favoured expression. Osmolality for aqueous solutions, is a bit higher than osmolarity, because the latter includes a little more volume (due to solutes) in the denominator, thus reducing the ratio osmoles/volume (L) with respect to osmoles/mass (kg) for the same osmoles of solute.

In any case, in spite of the preference for osmolality, both osmolality and osmolarity, measured for solutions in which the concentration and nature of each solute are exactly known, is different by the actual total molality and molarity (i.e. calculated osmolality and osmolarity), because of the non-ideal behavior of real solutions. This conversely means that the measured osmolalities, as the osmolarities, cannot be taken as the true values of total molality or molarity.