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.
Anatomy mOsm/l is a unit used in the nephron of the kidney in the effect called Countercurrent multiplication. The Loop of Henle's descending limb is permeable to water but not ions. The ascending limb is permeable to ions but not water. This causes a concentration gradient measured in mOsm/l.
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