What is meant by abnormal mass of solute? Discuss the factors which bring abnormality in the experimentally determined molecular masses of solutes using colligative properties.

Answer:

A molar mass that is either lower or higher than the expected or normal value is called as abnormal molar mass.

In order to account for all such abnormalities, introduced a factor
(i) known as van 't Hoff's factor, which represents the extent of association (or) dissociation of a solute.

Van't Hoff's factor is defined as the ratio of the observed colligative property to the calculated colligative property.

i = Observed colligative property / Calculated colligative property

Observed colligative property ∝ 1/Molar Mass

i = M_c/M_o

Van't Hoff's factor
(i) represents the extent of association (or) dissociation of a solute

i = Total number of moles of particles after association or dissociation / Number of moles of particles before association or disscussion

Experimentally determined molar mass is always lower than actual value for dissociation.

Molar Mass ∝ 1/Colligative Property

If the solute undergoes association in a solution, then the value of van 't Hoff's factor is less than one. If the solute undergoes dissociation then 'i' is greater than one.

Ex:

                       KCl → K⁺ + Cl^-

     1 molecule of KCl furnishes 2 ions in solution

i = Total number of moles of particles after dissociation / Number of moles of particles before dissociation

 i = 2/1 = 2

           2CH₃COOH ⇔(CH₃COOH)₂
          Ethanoic acid     Dimer of Ethanoic acid

i = Total number of moles of particles after association / Number of moles of particles before association

i = 1/2 =  0.5