Osmotic pressure

It is the flow of solvent molecules from the solvent or less concentrated solution to solution or more concentrated solution through a. semi-permeable membrane.

The osmotic pressure is developed by osmosis. It may be defined as, “The excess pressure which must be applied to the solution in order to just prevent the flow of the solvent into it through a semi permeable-membrane”.


“The excess of pressure which must be applied to the solution in order to increase its vapour pressure until it becomes equal to that of the solvent”.

According to Van’t Hoff, the solute molecules in dilute solution play the same role as is played by the gas molecules in gas, thus osmotic pressure being analogue of gas pressure.

Therefore we can say that,

\pi V = n ST = \dfrac{w}{m} ST


Where \pi = osmotic pressure, ‘V’ = volume in litre of solution, ‘n’ = number of moles of solute, ‘S’ or ‘R’ = a constant (0.0821 L atm K^{-1} mol^{-1}) and T = temperature in Kelvin.

Since n = \dfrac{w}{m} hence by knowing osmotic pressure of the solution molar mass (m) can be calculated.

Solutions which have the same osmotic pressure at the same temperature are said to be isotonic. In other words, isotonic solutions have the same molar concentration.

i. e.

\pi_1 = \pi_2 \\[3mm] \dfrac{w_1}{m_1 V_1} = \dfrac{w_2}{m_2 V_2}


When a plant cell is placed in solution of lower osmotic pressure (hypotonic) than that of the cell sap, water penetrates into it and it consequently swells, but when placed in a solution of higher osmotic pressure (hypertonic) than the cell sap, water is withdrawn from the cell and therefore shrinks. This phenomenon is known as Plasmolysis.

In isotonic solution osmosis, is not possible. Eg: 0.91% solution of pure NaCl is isotonic to human blood (R.B.C.)

If concn. of solution of NaCl > 0.91% R.B.C. will swell and may even burst.

  • If conc. Of NaCl < 0.91% R.B.C. will shrink, i.e., plasmolysis occurs
  • Berkeley, Hartley is best method to measure osmotic pressure due to less time and accurate value.

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