Osmole (unit)

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In chemistry, the osmole (Osm) is a non-SI unit of measurement that defines the number of moles of a chemical compound that contribute to a solution's osmotic pressure.

Osmolarity is a measure of the osmoles of solute per litre of solution, while the osmolality is a measure of the osmoles of solute per kilogram of solvent. Molarity and Osmolarity are not commonly used in osmometry because they are temperature dependent, that is water changes its volume with temperature. However, if the concentration is very low osmolarity and osmolality are considered equivalent. In calculations for these two measurements, salts are presumed to dissociate into their component ions. For example, a mole of glucose in solution is one osmole, whereas a mole of sodium chloride in solution is two osmoles (one mole of sodium and one mole of chloride). Both sodium and chloride ions affect the osmotic pressure of the solution.

The equation to determine the osmolality of a solution is given by $Osm = \phi\ n C$

where

Φ is the osmotic coefficient and accounts for the degree of dissociation of the solute. Φ is between 0 and 1 where 1 indicates 100% dissociation.
n is the number of particles into which a molecule dissociates. For example: Glucose equals 1 and NaCl equals 2.
C is the molal concentration of the solution

The units are Osm/kg

Osmolality can be measured using an osmometer which uses the principle of Freezing-point depression to determine the osmolality.