Question

In a solvent 50% of benzoic acid dimerises while rest ionises, determine molar mass of acid which is observed and also its van't Hoff factor.

Answer 1

Step 1:

$i=(1-\alpha )+\frac{\alpha }{n}$ $n=2$, as dimerisation occurs.

Also, $\alpha =\frac{50}{100}=0.5$

$\therefore i=(1-0.5)+\frac{0.5}{2}$

$i=0.75$

Step 2:

Vant Hoff factor=$\frac{Observedmolarmass}{Actualmolarmass}$

$\Rightarrow 0.75=\frac{Observed}{122g/mol}$

Observed molar mass = $\frac{122g{mol}^{-1}}{0.75}$

$=162.67g/mol$ .