Question

In an experiment to estimate the molecular weight of benzoic acid by elevation in boiling point method, the experimental value of molecular weight was double the actual value. Calculate the degree of association of dimer, if the elevation in B.P is $2^{\circ} C$.

• A. 1.0
• B. 0.5
• C. 0.9
• D. 2.0

Answer 1

Answer: (A)

$\because$ According to van't-Hoff volume $(i)$

$(i)=\frac{\text { Molecular mass (calculated) }}{\text { Molecular mass (experimental) }}$

Given, molecular mass (experimental) is double.

$\therefore i=\frac{1}{2}$

Also,

$\because (i)=(1-\alpha)+\frac{\alpha}{n}$

where, $\alpha=$ degree of association

and $n=$ number of particles associated.

(Here, $n=2, \because i=1 / 2$)

Therefore, $\frac{1}{2}=(1-\alpha)+\frac{\alpha}{2}$

$\frac{1}{2}=1-\frac{\alpha}{2}$

or, $\frac{\alpha}{2}=\frac{1}{2}$

or, $\alpha=1$