Gaseous ${\mathrm{N}}_2{\mathrm{O}}_4$ dissociates into gaseous ${\mathrm{NO}}_2$ according to the reaction ${\mathrm{N}}_2{\mathrm{O}}_4\left(g\right)\rightleftharpoons 2{\mathrm{NO}}_2\left(g\right)$. At 300 K and 1 atm pressure, the degree of dissociation of ${\mathrm{N}}_2{\mathrm{O}}_4$ is 0.2. If one mole of ${\mathrm{N}}_2{\mathrm{O}}_4$ gas is contained in a vessel, then the density of the equilibrium mixture is: