Osmosis occurs when there are different concentrations of a solvent on either side of a semipermeable membrane. In order for osmosis to occur, the membrane must be permeable to the solvent in question but not to the solute. Osmosis tends to equalize the concentrations of the solvent on either side of the membrane. If the solution on one side of the membrane is pure solvent and the membrane is impermeable to the solute, the concentrations on either side of the membrane can never be equal. However, at a certain point, the pressure of the solution against the membrane will prevent any further flow from the side with the pure solvent. The pressure at this point is called the osmotic pressure and is given by
where M is the molarity of the solution, R is the ideal gas constant, and T is the temperature in Kelvin. Osmotic pressure also depends only on the amount of solute and not on the type of solute and is therefore a colligative property.
