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)

$0.485$

Answer 2

$PCl_6(g) {->} PCl_3(g) + Cl_2(g)$
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 $= 1 + \alpha$
$= \frac{\text{Vapour density initial}}{\text{Vapour density at equilibrium}}$
$= \frac{104.25}{70.2} = 1.485$
$1 + \alpha = 1.485$
$\alpha = 0.485$