Four moles of $PC{l}_5$ are heated in a closed $4d{m}^3$ container to reach equilibrium at 400K. At equilibrium, 50% of $PC{l}_5$ is dissociated. What is the value of $K_c$ for the dissociation of $PC{l}_5$ into $PC{l}_{3}andC{l}_2$ at 400K?

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

No of moles of $PC{l}_5$ dissociated = 50% of 4 moles

                                                        = 2 moles

Amount at equilibrium

$PC{l}_5=4-2=2\text{moles}$

$PC{l}_3=2\text{moles}\text{=}\frac{2}{4}=0.5$

$C{l}_2=2\text{moles}\text{=}\frac{2}{4}=0.5$

$K_c=\frac{0.5\times 0.5}{0.5}=0.5M/L$