Osmole

One osmole is an amount of particles equal to Avogadro's number (6.02 × 1023).

Osmolarity

The amount of osmotically active particles present per litre of solution (mmol.l− 1).

Osmolality

The amount of osmotically active particles present per kilogram of solvent (mmol.kg− 1).

Osmotic pressure

The pressure exerted within a sealed system of solution in response to the presence of osmotically active particles on one side of a semipermeable membrane (kPa).

One osmole of solute exerts a pressure of 101.325 kPa when dissolved in 22.4 L of solvent at 0 °C.

Colligative properties

Those properties of a solution which vary according to the osmolarity of the solution. These are:

depression of freezing point. The freezing point of a solution is depressed by 1.86 °C per osmole of solute per kilogram of solvent

reduction of vapour pressure

elevation of boiling point

increase in osmotic pressure.

Raoult's law

The depression of freezing point or reduction of the vapour pressure of a solvent is proportional to the molar concentration of the solute.

Osmometer

An osmometer is a device used for measuring the osmolality of a solution. Solution is placed in the apparatus, which cools it rapidly to 0 °C and then supercools it more slowly to − 7 °C. This cooling is achieved by the Peltier effect (absorption of heat at the junction of two dissimilar metals as a voltage is applied), which is the reverse of the Seebeck effect.