Given,
No. of moles of PCl5=4mol
Volume of the flask=2litre
No. of moles of Cl2 left in the flask=0.8mol
Dissociation of PCl5 can be given as
PCl5⇌PCl3+Cl2
At t=0 4mol 0 0
At equilibrium (4−0.8)mol 0.8mol 0.8mol
To find equilibrium constantKc=[PCl3]×[Cl2][PCl5], we have to find the concentration of every component.
Concentration of the component = moles of the component/volume
Concentration of PCl5=4−0.8moles2=1.6mol/L
Concentration of PCl3=0.8moles2=0.4mol/L
Concentration of Cl2=0.8moles2=0.4mol/L
Substituting these values in the formula for equilibrium constant we get,
Kc=[PCl3]×[Cl2][PCl5]
Kc=0.4×0.41.6=0.1
Since, 0.1 can also be written as 1.0×10−1 , therefore option A is correct.