Question

The extent of dissociation of $PC{l}_5$ at a certain temperature is $20\%$ at one $atm$ pressure. Calculate the pressure at which this substance is half dissociated at the same temperature.

• A. $0.123$
• B. $0.246$
• C. $0.826$
• D. $0.111$

Answer 1

The correct option is A $0.123$

$PC{l}_5\rightleftharpoons PC{l}_3+C{l}_2$

case 1	$1-0.2$	$0.2$	$0.2$	P = 1 atm
case 2	$1-0.5$	$0.5$	$0.5$	P = $x$ atm

$K_p=\frac{\left(0.2/1.2\right)\left(0.2/1.2\right)}{0.8/1.2}=\frac{(0.5\times x/1.5)(0.5\times x/1.5)}{0.5\times x/1.5}$

$x=1.5\times 0.04/(1.2\times 0.8\times 0.5)=0.123$