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.