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osmotic pressure, Π

Excess pressure required to maintain osmotic equilibrium between a solution and the pure solvent separated by a membrane permeable only to the solvent:
Π = − R T V A ln a A
where V A, a A are the partial molar volume and activity of solvent A for an incompressible fluid. For ideal dilute solutions, Π = c B R T = ρ B R T M B, where entities B are individually moving solute molecules, ions, etc., regardless of their nature, c B, ρ B are the amount and mass concentration of the solutes, and M B is the mass average molar mass of the solutes. The amount is sometimes expressed in osmol (meaning a mole of osmotically active entities), but this usage and the corresponding term osmolarity are discouraged.
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IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
Last update: 2014-02-24; version: 2.3.3.
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