Question

A weak electrolyte $\left(A{B}_2\right)$ with formula mass 100 g / mole is dissociated in water upto 50%. The observed molar mass of the electrolyte is:

• A. $100$
• B. $\frac{200}{3}$
• C. $50$
• D. $25$

Answer 1

Answer: (C)

$50$

The relationship between the Van't Hoff factor and the degree of dissociation is $i=\frac{1-\alpha +n\alpha }{1}$

Here n is the number of ions obtained on the dissociation of 1 solute molecule.

Substitute values in the above expression.

$i=\frac{1-0.5+3\times 0.5}{1}=2$

The relationship between the Van't Hoff factor, the normal molar mass, and the observed molar mass is given below.

$i=\frac{\text{Normal molar mass}}{\text{observed molar mass}}$.

Substitute values in the above expression.

$2=\frac{100g/mol}{\text{observed molar mass}}$.

Hence, the observed molar mass is $\frac{100g/mol}{2}=50g/mol$.