Osmotic pressure is a measure of the extra pressure that has to be exerted to counteract osmosis. The osmotic pressure of a solution is the force that has to be exerted to halt osmosis. If a U-shaped glass tube were divided in two by a semipermeable membrane and filled with two solutions the osmotic pressure could be shown. If a solution were on the left-hand side of the membrane and pure solvent were on the right-hand side then movement would occur from the right to the left.
This movement could be halted by applying pressure to the left hand arm (the one containing the solution). The pressure exerted is the osmotic pressure. The osmotic pressure obeys a law similar in form to that of the ideal gas law, PiV = nRT, where Pi is the osmotic pressure, V is the volume of the solution, n is the number of moles of solute, R is the ideal gas constant, and T is the absolute temperature. Since the number of moles of solute divided by the volume of the solution is the molarity (M) of the solution this equation can be rewritten as Pi = MRT. If two solutions are separated by a semipermeable membrane and they have the same osmotic pressure they are said to be isotonic. If one solution has a lower osmotic pressure it is said to hypotonic to the more concentrated solution, which is termed the hypertonic solution.
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