The vapour density of an equilibrium mixture of PCl5, PCl3 and Cl2 is found to be 80 (calculated value for undissociated PCl5 is 104.25). The degree of dissociation of PCl5 is

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The vapour density of an equilibrium mixture of $PCl X_{5}$ , $PCl X_{3}$ and $Cl X_{2}$ is found to be $80$ (calculated value for undissociated $PCl X_{5}$ is $104.25$ ). The degree of dissociation of $PCl X_{5}$ is

α = (D - d) / d = (104.25 - 80) /80 = 0.303

, i.e. about 0.30. Dividing by

D

instead of

d

gives 0.23.