What is the degree of dissociation of sodium chloride, if the molar mass determined by a cryoscopic method…
What is the degree of dissociation of sodium chloride, if the molar mass determined by a cryoscopic method was found to be $31.80 \mathrm{~g} \mathrm{~mol}^{-1}$ [Atomic mass $\mathrm{Na}=23 \mathrm{~g} \mathrm{~mol}^{-1} \mathrm{Cl}=35.5 \mathrm{~g}$ $\left.\mathrm{mol}^{-1}ight] ?$
0.85
0.83
0.84
0.82
Solution
Let $\alpha$ be the give of dissociati $\mathrm{n}$ then
Van't Hoff's factor $\mathrm{i}=\frac{1-\alpha+\alpha+\alpha}{1}=1+\alpha$
Again Van't Hoff's factor
$=\frac{\text { Normal mol. wt }}{\text { Observed mol. wt }}=\frac{58.5}{31.8}=1.839 \approx 1.84$
Equating to both values of $\mathrm{i}, \quad \therefore 1+\alpha=1.84$
$\therefore \alpha=0.84$