Question

The experimental values of colligative properties of many solutes in solution resembles calculated values of colligative properties.
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 $\text{KCl, NaCl}$ etc. in $H_{2}O$.
(ii) Association of solute: It decreases the colligative properties.
e.g.: Dimerisation of acetic acid in benzene.

If the degree of dissociation of an electrolyte $A_2{B}_3$ is 25% in a solvent, then:

• A. Normal boiling point = Experimental boiling point
• B. Normal freezing point > Experimental freezing point
• C. Normal osmotic pressure $=\frac{1}{2}$ Experimental osmotic pressure
• D. Normal molecular weight $=\frac{1}{4}$ Experimental molecular weight

Answer 1

The correct option is C Normal osmotic pressure $=\frac{1}{2}$ Experimental osmotic pressure

Electrolyte $A_2{B}_3$ is 25% dissociated.
$A_2{B}_3\rightleftharpoons 2A^{3+}+3B^{2-}$
$i=1+(5-1)\left(0.25\right)=2$
$\frac{{\pi }_{obs}}{{\pi }_{cal}}$ = 2
${\pi }_{cal}$ = $\frac{1}{2}$${\pi }_{obs}$
normal osmotic pressure = $\frac{1}{2}$(observed osmotic pressure)