a = (D-d)/(n-1)d
Here a is alpha()degree of dissociation), D=vapour density before dissociation=M react/2
d=vapour density after dissociation=M obs/2
n =number of moles of product obtained by one mole of reactant
Now here D=208.5/2 = 104.25
And d= 104.25(given)
And n = 2/1=2
On putting theSe values we get a = 0