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25.  When heating PCl5 then it decompose PCl3 and Cl2 in form of gas, The vapour density of gas mixture is 70.2 and 57.9 at 200 °  C and 250 ° C. The degree of dissociation of PCl5 at 200 ° C and 250 ° C is

(1) 48.50% & 80%                                                (2) 60% & 70%
(3) 70% & 80%                                                     (4) 80% & 90%

Dear Student,

We know that when PCl5 is heated it decompose to give PCl3 and Cl2
PCl5 →     ​PCl3 + ​Cl2
Given,
The vapour density at equilibrium at 200 and 250 C.The initial Vapour  Density at both the temperatutre MPCl5/2
therefore,initial V.D.=(31+5*35.5)/2=104.5
V..D. at equilibrium at 200 C =70.2 (given)
total mole at equilibrium /total moles initial =1+α=initial V.D./V.D. at equilibrium 
1+α=104.5/70.2
       =1.485
α=0.485
At 250ο ​C
1+​α=104.25/57.9
      =1.8
α=0.80
Regards

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