The maximum pressure which is developed in a solution, when it is separated from its pure solvent by a semipermeable membrane is called osmotic pressure. Osmotic pressure is not an actual physical pressure that exists in a solution, but a potential pressure that can be developed only when the solution is separated from a solvent by a semipermeable membrane. Since a solution in a container does not create an osmotic pressure unless a semipermeable membrane separates it from pure water. Hence osmotic pressure of a solution may also be called as osmotic potential.
Osmotic pressure develops due to difference in the diffusion pressure of water on the two sides of a semipermeable membrane. It is equivalent to the pressure, which is applied externally in order to prevent the flow of solvent into the solution due to osmosis. The osmotic pressure of a solution can be measured with the help of an apparatus called osmometer.
Osmotic pressure is directly proportional to the number of molecules in a given solution. More is the number of solute molecules in a solution; more would be its osmotic pressure. Higher number of solute molecules in a solution lowers its free energy .This results in its increased osmotic pressure. The osmotic pressure of a solution is directly proportional to the concentration of the solution. The value of osmotic pressure is always positive.
The value of osmotic pressure of a solution can be calculated by the following formula
OP= CX RX T where C is the concentration of solute particles in moles per liter, R is the gas constant and T is the absolute temperature.