<P> Osmotic concentration, formerly known as osmolarity, is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution (osmol / L or Osm / L). The osmolarity of a solution is usually expressed as Osm / L (pronounced "osmolar"), in the same way that the molarity of a solution is expressed as "M" (pronounced "molar"). Whereas molarity measures the number of moles of solute per unit volume of solution, osmolarity measures the number of osmoles of solute particles per unit volume of solution . This value allows the measurement of the osmotic pressure of a solution and the determination of how the solvent will diffuse across a semipermeable membrane (osmosis) separating two solutions of different osmotic concentration . </P> <P> The unit of osmotic concentration is the osmole . This is non-SI unit of measurement that defines the number of moles of solute that contribute to the osmotic pressure of a solution . A milliosmole (mOsm) is 1 / 1,000 of an osmole . A microosmole (μOsm) (also spelled micro-osmole) is 1 / 1,000,000 of an osmole . </P>

The measurement of moles of solute per liter of solution is known as