Question

When $ PCl_5 $ is heated it gasifies and dissociates into $ PCl_3 $ and $ Cl_2 $ . The density of the gas mixture at $ 200^{\circ}C $ is $ 70.2 $ . What is the degree of dissociation of $ PCl_5 $ at $ 200^{\circ}C $ .

• A. $ 0.485 $
• B. $ 0.242 $
• C. $ 0.845 $
• D. $ 0.542 $

Answer 1

Answer: (A)

$PCl_6(g) \rightarrow PCl_3(g) + Cl_2(g)$
The initial vapour density will be same, it would be $M_{PCl_5}/2$
$\therefore $ Initial vapour density
$= (31 + 5 \times 35. 5) /2 =104.25$
Vapour density at equilibrium at $200^{\circ}C = 70.2$
$\therefore $ Total moles at equilibrium
Total moles initial $= 1 + \alpha$
$ = \frac{\text{Vapour density initial}}{\text{Vapour density at equilibrium}}$
$ = \frac{104.25}{70.2} = 1.485$
$ 1 + \alpha = 1.485$
$ \alpha = 0.485$