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What Are Osmolarity and Osmolality?

Osmolarity and osmolality are units of solute concentration that are often used in reference to biochemistry and body fluids. Learn what osmolarity and osmolality are and how to express them.


Both osmolarity and osmolality are defined in terms of osmoles. An osmole is a unit of measurement that describes the number of moles of a compound that contribute to the osmotic pressure of a chemical solution.
The osmole is related to osmosis and is used in reference to solution where osmotic pressure is important, such as blood and urine.


Osmolarity is defined as the number of osmoles of solute per liter (L) of solution. It is expressed in terms of osmol/L or Osm/L.
A 1 mol/L NaCl solution has an osmolarity of 2 osmol/L. A mole of NaCl dissociates fully in water to yield two moles of particles: Na+ ions and Cl- ions. Each mole of NaCl becomes two osmoles in solution.


Osmolality is defined as the number of osmoles of solute per kilogram of solvent. It is expressed in terms of osmol/kg or Osm/kg.

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