osmoles

An osmole is a unit of amount of substance used to express the quantity of osmotically active particles in a system. One osmole corresponds to one mole of particles that contribute to osmotic pressure under given conditions, approximately Avogadro’s number of entities. For solutes that dissociate or associate in solution, the effective number of particles is modified by the van’t Hoff factor i, so the osmole count can be i times the amount of solute.

Osmolarity and osmolality are the practical measures related to osmoles. Osmolarity expresses osmoles per liter of solution (Osm/L), while osmolality expresses osmoles per kilogram of solvent (Osm/kg). In dilute solutions, osmolarity and osmolality are close in value. Examples: dissolving sodium chloride yields about 2 osmoles per mole (due to dissociation into Na+ and Cl−, i ≈ 2); glucose yields 1 osmole per mole (no dissociation); calcium chloride yields about 3 Osm per mole (i ≈ 3).

In clinical and laboratory contexts, the term osmole is used less frequently than osmolarity or osmolality, with measurements often reported as milliosmoles per liter (mOsm/L) or milliosmoles per kilogram (mOsm/kg). The osmole is a non-SI unit and is primarily of historical and physiologic relevance; 1 Osmol equals 1000 milliosmoles.