<a href=http://en.wikipedia.org/wiki/Osmolarity>http://en.wikipedia.org/wiki/Osmolarity</a>;

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.

Osmolality is a measure of the osmoles of solute per kilogram of solvent (osmol/kg or Osm/kg).

Molarity and osmolarity are not commonly used in osmometry because they are temperature dependent. This is because water changes its volume with temperature (See: Vapour pressure of water). However, if the concentration of solutes is very low, osmolarity and osmolality are considered equivalent.