"At $127^{\circ} \mathrm{C}$ and 1 atm pressure, $\mathrm{PCl}_{5}(\mathrm{~g})$ is partially dissociated into $\mathrm{PCl}_{3}(\mathrm{~g})$ and $\mathrm{Cl}_{2}(\mathrm{~g})$ as $\mathrm{PCl}_{5}(\mathrm{~g}) \rightleftharpoons$ $\mathrm{PCl}_{3}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g}) .$ The density of the equilibrium mixture is $3.5 \mathrm{~g} / \mathrm{L}$. Percentage dissociation of $\mathrm{PCl}_{5}$ is $(R=0.08 \mathrm{~L}-\mathrm{atm} / \mathrm{K}-\mathrm{mol}, \mathrm{P}=31, \mathrm{Cl}=35.5)$