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(b) Association :
(i) In some non-polar solvents, the solute molecules undergo association, i.e. two,
three or even more molecules exist in combination with each other to form bigger
(ii) Therefore, the total number of molecules in solution become less than the
number of molecules of the substance added. This leads to a decrease in
the value of colligative properties because colligative properties is proportional
to no. of solute particles.
(iii) Since the colligative property is inversely proportional to the molar mass of the
solute, the molar mass of solute in such cases will be greater than normal.
(iv) For example, in benzene solvent solutes like acetic acid, benzoic acid etc. exist
as dimers. This is due to hydrogen bonding between these molecules.
(CH 3COOH) 2
2CH 3COOH (C 6 H 5COOH) 2
2C 6 H 5COOH (j) vant Hoff factor :
1. This factor is used to express the extent of dissociation or association of the solutes in
their solutions.
2. It defines as the ratio of observed colligative property produced by a given concentration
of electrolyte solution to the property observed for the same concentration of non electrolyte
3. Therefore,
observed value of colligative property
i = theoretical value of colligative property
Since colligative property α number of solute particles present in the solution. Hence,
number of solute particles present in solution
theoretical number of solute particles
Similarly, colligative property α
molecular mass
theoretical molecular mass
i = observed molecular mass
If i = 1, solute molecules are in normal state and no association or dissociation has taken
If i > 1 , the solute molecules are dissociated in the solution.
If i < 1, the solute molecules are associated in the solution.
Unique Solutions ®
S.Y.J.C. Science - Chemistry - Part I
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