Osmosis and Osmotic Pressure of Solutions

A semipermeable membrane allows some substances to pass through but not others. This movement of solvent molecules across a semipermeable membrane, to a solution with higher solute concentration, is called osmosis.

Consider a U-shaped tube containing pure water on the left and a sugar solution on the right separated by a semipermeable membrane.

Water molecules will flow to the sugar solution at a faster rate than the reverse to try to establish a concentration equilibrium.

As water flows to the right, the level of liquid in the two arms becomes unequal.

Eventually, the added weight of the excess water on the right creates sufficient pressure to stop osmosis.

The minimum pressure required to halt osmosis is called the osmotic pressure. It is a colligative property that is dependent on the solute concentration in the solution.

As the concentration of the solute increases, the osmotic pressure increases proportionally.

Osmotic pressure, π, can be calculated by multiplying the molarity of the solute by the temperature in kelvin and the ideal gas constant R, 0.0821 liter-atmosphere per mole kelvin.

If the concentration of the sugar solution is 1.00 molar, then at 25 °C or 298 K, the osm