The vapour density of undecomposed N2O4 is 46. When heated, vapour density decreases to 24.5 due to its dissociation to NO2. The % dissociation of N2O4 at the final temperature is

N2O4 ⇌  2NO21                 0                        at initial1-α           2α                    at equilibrium∴     VDinitialVDfinal = nfinalninitial 4625.4  =  25.446= 1+α11.8=1 + α  ⇒  α   =  0.8or      80%