Simply put, osmolality is a measurement of the total number of solutes in a liquid solution expressed in osmoles of solute particles per kilogram of solvent. When solutes are introduced in a solvent, the resulting solution differs from the initial solvent in several ways.

Boiling PointBoiling Point Elevation 0.52°C
Freezing PointFreezing Point Depression 1.86°C
Osmotic PressureOsmotic Pressure Elevation 17,000 mm Hg
Vapor PressureVapor Pressure Depression 0.3mm Hg

The resulting changes in these properties are not proportional to the weight, size or shape of the dissolved particles, but only to their molal concentration. Osmolality therefore is an ideal measurement to estimate the total concentration of solutes in a near limitless variety of liquid sample matrices, including blood, serum, plasma, urine, milk, cell culture media and almost all forms of aqueous based solutions.


For a simple chemical, such as urea, the effect is related to the total number of moles of urea in solution. For a chemical compound which can dissociate, such as sodium chloride, both the sodium and the chloride ions will contribute to these colligative properties. Therefore one mole of sodium chloride will have twice the effect as will one mole of urea. Using freezing point depression as an example:

  • One mole of urea will depress the freezing point of the solution by 1.86° C
  • One mole of sodium chloride NaCl (1mole Na+ + 1 mole Cl-) will depress the freezing point by 3.72° C
  • One mole of calcium chloride CaCl2 (1 mole Ca++ + 2mole Cl-) will depress the freezing point by 5.58° C
  • The actual mass of the particle is irrelevant, since a small molecule will exert the same effect as a large molecule.

Osmolality Equation

Osmolality is the number of Osmols of solute particles per kilogram of pure solvent. Since most ionic species do not completely dissociate, osmolality is a unit of concentration, which takes into account the dissociative effect. Osmolality is usually expressed in mOsm/kg H20. One milliosmol (mOsm) is 10-3 osmols. The osmolality equation is:

Osmolality = ΦnC = osmol / kg H20


Φ = osmotic coefficient, which accounts for the degree of molecular dissociation

n = number of particles into which a particle can dissociate

C = molal concentration of the solution