Van't Hoff Factor and Abnormal Molar Mass

What is the Van’t Hoff Factor?

The Van’t Hoff factor offers insight on the effect of solutes on the colligative properties of solutions. It is denoted by the symbol ‘i’. The Van’t Hoff factor can be defined as the ratio of the concentration of particles formed when a substance is dissolved to the concentration of the substance by mass.

The extent to which a  substance associates or dissociates in a solution is described by the Van’t Hoff factor. For example, when a non-electrolytic substance is dissolved in water, the value of i is generally 1. However, when an ionic compound forms a solution in water, the value of i is equal to the total number of ions present in one formula unit of the substance.

For example, the Van’t Hoff factor of CaCl2 is ideally 3, since it dissociates into one Ca2+ ion and two Cl ions. However, some of these ions associate with each other in the solution, leading to a decrease in the total number of particles in the solution.

This factor is named after the Dutch physical chemist Jacobus Henricus Van’t Hoff, who won the first Nobel Prize in chemistry. It is important to note that the measured value of the Van’t Hoff factor for electrolytic solutions is generally lower than the predicted value (due to the pairing of ions). The greater the charge on the ions, the greater the deviation.

Effects of Association/Dissociation

  • Association is the joining of two or more particles to form one entity.
  • An example of the association of two particles is the dimerization of carboxylic acids when dissolved in benzene.
  • Dissociation refers to the splitting of a molecule into multiple ionic entities.
  • For example, sodium chloride (NaCl) dissociates into Na+ and Cl ions when dissolved in water.

The effects of the association or dissociation of a solute on the solution, its colligative properties, and the Van’t Hoff factor are tabulated below.

Association Dissociation
Observed molar mass is greater than the predicted value The observed value of molar mass is lesser than the normal value.
The value of the Van’t Hoff factor is less than one. The value of i is greater than one.
The values of the colligative properties are lower than expected. Example: reduced boiling point and freezing point. Higher values of colligative properties are observed. For example, higher osmotic pressure and boiling point.

Abnormal Molar Masses

The theoretical values of molecular mass, when calculated from the colligative properties of solutions, are sometimes found to differ from the experimentally obtained values. These values are often referred to as abnormal molar masses.

Van’t Hoff explained that when solutes are dissolved in a solvent they dissociate into ions. Since colligative properties depend only on the number of solute particles, the dissociation of solute molecules into ions results in an increase in the number of particles and hence affects the colligative properties.

When 1 mole of NaCl is dissolved in 1 Kg of water, if all the molecules of NaCl dissociate in water, there will be 1 mole of Cl ions and 1 mole of Na+ ions in the resulting solution (a total of 2 moles of ions in the solution). But while calculating the molar mass using the colligative properties, we consider only 1 mol of NaCl to be present in the solution.

Some of the substances tend to associate in aqueous state and for such molecules, the number of ions/molecules present in the solution is less than the actual number of molecules. So, for those substances that dissociate in solution, the observed molar mass will always be less than the real mass and for those substances that associate in solutions, the real mass will always be less than the observed molar mass.

The abnormality in the molecular mass can be explained as follows:

  • The dissociation of solute molecules into multiple ions results in an increase in the number of particles. This, in turn, increases the colligative properties of the solution.
  • Since the molar mass is inversely proportional to the colligative properties, its value tends to be lower than expected.
  • When solute particles associate with each other, the total number of particles in the solution decreases, leading to a decrease in the colligative properties.
  • In this case, the molar mass values obtained are higher than expected.

Thus, the Van’t Hoff factor is briefly described in this article. The abnormal molar mass values obtained from the colligative properties of solutions are also explained with the help of this factor. To learn more about this topic and other related topics, such as Raoult’s Law, register with BYJU’S and download the mobile application on your smartphone.

Practise This Question

The boiling point of 0.2 mol kg1solution of X in water is greater than equimolal solution of Y in water. Which one of the following statements  is true in this case?