Osmotic pressure is another example of a colligative property, one that depends only on the number of solute molecules or ions. It's a bit less obvious than something like boiling point elevation, but not hard to understand.

Consider making up a solution of sugar in water. Place this solution in a bag made of a semipermeable membrane. This is a membrane which has many small holes in it which allows the solvent to flow through but not the solute. Place the full bag in a bucket of pure water: the pure water will begin to diffuse into the bag of sugar water, causing a pressure in the bag. This is known as osmotic pressure.

A simple example can be seen when making a pickle. A cucumber is placed in a brine solution and the skin of the cucumber acts like a semipermeable membrane, allowing the water in the cucumber to flow out into the brine solution. You end up with a shriveled cucumber. The opposite process is used to purify water: place impure water in the bag, then put a large external pressure (Greater than the osmotic pressure forcing water into the bag.) on the bag, forcing the water molecules across the barrier while leaving the impurities behind.

The osmotic pressure is given by the equation

P = MRT

When working with ionic solutes in water and other polar solvents, one must be careful to take into consideration the fact that the ion concentration is higher than the concentration of the solute. For example, table salt, NaCl dissolves in water to form ions

NaCl(s) -> Na₊(aq) + Cl^-(aq)

Example: What is the osmotic pressure of a 1.00 molar sugar solution in water at 25^oC?

Solution: Simply use the above equation and substitute in the proper values

P = MRT

P = 1.00 mol/L * 0.0812 (L*atm/mol*K) * 298 K

P = 24.5 atm

Information provided by: http://learn.chem.vt.edu