According to the reaction gaseous N2O4 dissociates into gaseous NO2: N2O4(g)⇌2NO2(g). At 300K and 1 atm pressure, the degree of dissociation of N2O4 is 0.2. If one mole of N2O4 gas is contained in a vessel, then the density of the equilibrium mixture is:

N2O4⇌2NO2
Initial moles 1 0
Change -0.2 +0.4
Equilibrium moles 1-0.2=0.8 0.4
The total number of moles at equilibrium = 0.8 + 0.4 = 1.2 moles
Using the ideal gas equation, PV = nRT
V = nRT/P = (1.2 moles × 0.0821 L atm/mol K × 300 K) / 1 atm = 29.556 L
Total mass of the mixture = (0.8 moles × 92 g/mol) + (0.4 moles × 46 g/mol) = 73.6 + 18.4 = 92 g
Density = mass/volume = 92 g / 29.556 L ≈ 3.11 g/L
The molar masses of NO2 and N2O4 are 46 g/mol and 92 g/mol respectively.