N2O4 (g)            2NO2(g)Initial 1 mole                 0At eqn. 1 - 0.5           2 x 0.5       = 0.5 mol            = 1 molTotal number of moles = 0.5 + 1  = 1.5 mol
According to law of chemical equilibrium 
We know,         Substituting the values in equation (1), we get