In chemistry, the osmole (Osm or osmol) is a non-SI unit of measurement that defines the number of moles of solute that contribute to the osmotic pressure of a solution. 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-).
The difference between osmolarity and molarity can be easily explained by an example. A physiological saline solution can be made by 9 g of sodium chloride dissolved in 1 liter of water, or 50 grams of glucose in 1 liter. Salt has a molar mass of 58.44 g/mol, glucose of 180.15 g/mol. 50 grams of glucose results in 278 millimoles of glucose. 9 g of sodium chloride correspond to 154 millimoles of sodium chloride. The osmolarity of the solution of sodium chloride, however, is 308 milliosmoles / liter. This difference is due to the number of particles after solvation: one molecule of sodium chloride in water splits into two ions. Glucose, however, remains one molecule after dissolvation. As a result, the osmolarity of 50 g of glucose in 1 liter of water is approximately equal to 9 g of sodium chloride in 1 liter of water.
A milliosmole (mOsm) is 1/1,000 of an osmole. A microosmole (μOsm) (also spelled micro-osmole) is 1/1,000,000 of an osmole.
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