Let the molecular mass of the mixture equilibrium be Mmix.
Applying the relation,
Mmix=dRTP=0.925×0.0821×9001=68.348
Molecular mass of SO3=80
Vapour density of O3,D=802=40
vapour density of mixture, d=68.3482=34.174
Let the degree of dissociation be x
x=D−d(n−1)d=40−34.174(32−1)×34.174=0.34
or x=34% dissociated
i.e., SO3 is 34% dissociated.