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Abnormal Colligative Properties

Why is the ma...

Question

Why is the mass determined by measuring a colligative property in case of some solutes abnormal? Discuss it with the help of van't Hoff factor.

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Solution

Certain compounds when dissolved in suitable solvents either dissociate or associate.e.g., ethanoic acid dimerises in benzene due to H-bonding, while in water, it dissociates and forms ions.
As a result the number of chemical species in solution increases or decreases as compared to the number of chemical species of solute added to form the solution.
Since, the magnitude of colligative property depends on the number of solute particles, it is expected that the molar mass determined on the basis of colligative properties will either higher or lower than the expected value or the normal value and is called abnormal molar mass.
In order to account for the extent of dissociation or association of molecules in solution, van't Hoff introduced a factor, i, known as the van't Hoff factor.
$i = \frac{e x p e c t e d m o l a r m a s s}{a b n o r m a l m o l a r m a s s} = \frac{o b s e r v e d c o l l i g a t i v e p r o p e r t y}{c a l c u l a t e d c o l l i g a t i v e p r o p e r t y} = \frac{t o t a l n u m b e r o f m o l e s o f p a r t i c l e s a f t e r a s s o c i a t i o n o r d i s s o c i a t i o n}{t o t a l n u m b e r o f m o l e s o f p a r t i c l e s b e f o r e a s s o c i a t i o n o r d i s s o c i a t i o n}$

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(i) Dissociation of solute: It increases the colligative properties.
e.g.: Dissociation of

KCl, NaCl

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(ii) Association of solute: It decreases the colligative properties.
e.g.: Dimerisation of acetic acid in benzene.

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(i) Dissociation of solute: It increases the colligative properties.
e.g.: Dissociation of

KCl, NaCl

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(ii) Association of solute: It decreases the colligative properties.
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However in some cases, the experimental values of colligative properties differ widely than those obtained by calculations. Such experimental values of colligative properties are known as abnormal values of colligative properties. The causes for abnormal values of colligative properties are:
(i) Dissociation of solute: It increases the colligative properties.
e.g.: Dissociation of

KCl, NaCl

H_{2} O

.
(ii) Association of solute: It decreases the colligative properties.
e.g.: Dimerisation of acetic acid in benzene.

Four different

100 ml

solutions are prepared by mixing 1 gram each of

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K_{4} [F e (C N)_{6}]

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