Osmole (unit)

This article is about the osmole unit. For the osmolarity, see Osmolarity.

In chemistry, the osmole (Osm or osmol) 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. The term comes from the phenomenon of osmosis, and is typically used for osmotically-active solutions. For example, a solution of 1 mol/L NaCl corresponds to an osmolarity of 2 osmol/L. The NaCl salt particle dissociates fully in water to become two separate particles: an Na⁺ ion and a Cl^- ion. Therefore, each mole of NaCl becomes two osmoles in solution, one mole of Na⁺ and one mole of Cl^-. Similarly, a solution of 1 mol/L CaCl₂, gives a solution of 3 osmol/L (Ca²⁺ and 2 Cl^-).

A strict definition of an osmole is the number of osmotically active particles that, when dissolved in 22.4 L of solvent at 0 degrees Celsius, exert an osmotic pressure of 1 atmosphere. This is equivalent to the observation that 1 mole of a perfect gas occupying a volume of 22.4 L exerts a pressure of 1 atmosphere.

1 osmole = 1 mole of osmotically active particles = o(n)(C),

where o is the osmotic coefficient, n the number of particles resulting from dissociation of each molecule in solution and C the concentration of each molecule in mol/kg of water.

A milliosmole (mOsm) is 1/1000 of an osmole. A microosmole (μOsm) (also spelled micro-osmole) is 1/1,000,000 of an osmole.