Colligative properties are properties which depend only on the number of particles of solute in a solution, and not on the identity of the solute. They apply only to solutions and are useful for determining many of their key properties. The most common of these are freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure.
To illustrate this concept, let’s look at two solutions, one of NaCl and one of KI. NaCl and KI both dissociate into two ions, so the number of solute particles in the solutions would be the same. As long as the concentrations of the two solutions are the same, the two solutions would have the same freezing and boiling points because NaCl and KI have an equal number of solute particles. The ratio of the moles of particles in solution to the moles of solute dissolved is expressed as the van ‘t Hoff factor, i. The expected value for i can be calculated simply by noting the number of ions per formula unit. For NaCl and KI, i would be 2; for K3PO4, i would be 4; and for Fe2(SO4)3 , i would be 5.
The van ‘t Hoff factor is most important for electrolyte solutions, or solutions that contain ions. It is essential in calculating freezing point depression, boiling point elevation, and osmotic pressure for electrolyte solutions. Because colligative properties are directly related to the number of solute particles in a solution, they are very useful for describing the nature of a solute after it is dissolved and also for determining the molar masses of substances.