Osmotic Pressure

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Equation / Last modified by ekskekel on 2016/09/11 20:03
`pi = `
ekskekel.Osmotic Pressure

The Osmotic Pressure formula, π = MRT, represents the pressure required to prevent a pure solvent from passing into a solution through osmosis, as shown in the figure below. Osmotic pressure plays an important role in cell regulation. It helps cells prevent too much water from either escaping or entering the cell, thus maintaining a homeostasis. Osmotic pressure can also be used to estimate molecular weights, particularly for higher weight molecules. 

The input variables are:

  • molar concentration of the solution (M)
  • ideal gas constant (R)
  • temperature (T)

The default units are moles/liter for molar concentration and Kelvin for temperature. The Ideal Gas Constant (R) uses the value 0.08206 L*atm/mol*K.



 Osmotic pressure increases with temperature because temperature affects the number of solvent-membrane collisions per minute. It also increases with molarity because molarity leads to a higher or lower number of molecules hitting the membrane. A higher molarity leads to a stronger drive to equalize the concentration and increase disorder in the solution.

Supplemental Materials

Khan Academy - Diffusion and Osmosis

Video Demonstration


Whitten, et al. "Chemistry" 10th Edition. Pp. 530

Wikipedia - Osmotic Pressure


This equation, Osmotic Pressure, is listed in 3 Collections.