Gaseous $N_2O_4$ dissociates into gaseous $NO_2$ according to the reaction $ N_2O_4 (g) \leftrightharpoons 2NO_2 (g)$ At $300\; K$ and $1\; atm \;pressure$, the degree of dissociation of $N_2O_4$ is $0.2$. If one mole of $N_2O_4$ gas is contained in a vessel, then the density of the equilibrium mixture is :
