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OSMOLARITY (OsM)
Is the sum of the molar (M) concentrations of all the solute particles in solution. This ignores the fact that the effective concentration is reduced by interactions between solute particles, but is a good approximation for dilute solutions.
Example
The osmolarity of a 1 M NaCl solution is 2 Osm. This is because in solution 1 mole of NaCl dissociates to give 1 mole of Na⁺ ions and 1 mole of Cl^- ions.