## osmotic pressure, $$\mathit{\Pi}$$

https://doi.org/10.1351/goldbook.O04344
Excess pressure required to maintain osmotic equilibrium between a solution and the pure solvent separated by a membrane permeable only to the solvent: $\mathit{\Pi} = -\frac{R\ T}{V_{\text{A}}}\ \ln a_{\text{A}}$ where $$V_{\mathrm{A}}$$, $$a_{\mathrm{A}}$$ are the partial molar volume and activity of solvent $$\text{A}$$ for an incompressible fluid. For ideal dilute solutions, $$\mathit{\Pi} = c_{\text{B}}\ R\ T=\rho _{\text{B}}\ \frac{R\ T}{M_{\text{B}}}$$, where entities $$\text{B}$$ are individually moving solute molecules, ions, etc., regardless of their nature, $$c_{\text{B}}$$, $$\rho _{\mathrm{B}}$$ are the amount and mass concentration of the solutes, and $$M_{\mathrm{B}}$$ 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.
Sources:
Green Book,2nd ed., p. 51 (https://www.cintegrado.com.br/site/documentos/green_book_2ed.pdf)
Green Book,3rd ed., p. 59 (https://doi.org/10.1039/9781847557889)