In a 500 ml flask, the degree of dissociation of ${\mathrm{PCl}}_5$ at equilibrium is 40% when the initial amount taken  is 5 moles. The value of equilibrium constant in moles/lit  for the decomposition of ${\mathrm{PCl}}_5$ is ( Given reaction, ${\mathrm{PCl}}_5\left(\mathrm{g}\right)\rightleftharpoons {\mathrm{PCl}}_3\left(\mathrm{g}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)$ )

V_vessel = 0.5 lit

Degree of decomposition of PCl₅ =

.

For every 1 mole PCl₅ taken 0.4 moles decomposes.

For every 5 moles PCl₅ taken 'X' moles decomposes.

Number of moles of PCl₅ decomposed = 2

For every 2 moles of PCl₅ decomposing 2 moles each of PCl₃ and Cl₂ are formed.

Stoichiometry
Initial moles	5		-	-
Moles at equilibrium	(5 - 2)		2	2
Equilibrium concentration