CBSE Class 12 Chemistry Notes: Solutions – Osmosis and Osmotic Pressure


In osmosis, there is a net flow of solvent molecules from the solvent to the solution or from a less concentrated solution to a more concentrated solution across a semipermeable membrane (membranes of animal origin, membranes made from

Cu2 [Fe (CN) 6], Ca3 (PO4)2 etc.)

Osmosis was first observed by Abbe Nollet in 1748.

Osmotic Pressure

Osmotic pressure is the equilibrium hydrostatic pressure of the column set up as a result of osmosis. It is the minimum pressure that must be applied on the solution to prevent the entry of the solvent into the solution through the semi­permeable membrane. It is the minimum pressure needed to apply on a solution to make its vapour pressure equal to vapour pressure of the solvent.

  • It is denoted by P or π.
  • It is measured by Pfeiffer’s method, Berkely­Hartley’s method, Townsend’s method.

Van’t Hoff equation of osmotic pressure

π = CST
Here, π = Osmotic pressure
C = Concentration of solution in mol/L
S or R = Solution constant

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