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Van’t Hoff Factor

Colligative properties depend on the number of particles of the solute. However, in some cases where the solute associates or dissociates in solution, abnormal results for colligative properties are obtained.

Dissociation

Let us consider the case of acetic acid dissolved in water. Let the number of moles of acetic acid dissolved be n.
 
 
 
 
Theoretical colligative property (for example, ΔTb) (based on the number of moles of CH3COOH added):
 
Description: 45319.png
 
where W is the mass of solvent in gram.
 
Experimental colligative property would be given by
 
Description: 45328.png
Description: 45336.png
= (1 + α) = I, Van’t Hoff factor

Association

Let us consider the case of acetic acid dissolved in benzene. Let the number of moles of acetic acid dissolved by n and it forms dimer in benzene.
 
 
Theoretical colligative property (for example, ΔTb) (based on the number of moles of CH3COOH added):
 
ΔTb = Kb m = Kb × Description: 45347.png
 
where W is the mass of solvent in gram. Experimental colligative property would be given by
 
Description: 45355.png
 
Description: 45364.png
 
Description: 45372.png Van’t Hoff factor
 
Therefore, for dissociation i > 1 and for association i < 1. When a solute neither dissociates nor associates, the Van’t Hoff factor is equal to 1.




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