2NO2(g) ⇌ N2O4(g)
\[\begin{aligned} K &= \frac {[\mathrm{N_2O_4}]}{[\mathrm{NO_2}]^2} \\ [\mathrm{NO_2}]^2 &= \frac {[\mathrm{N_2O_4}]}{K} \\ [\mathrm{NO_2}] &= \sqrt{\frac {[\mathrm{N_2O_4}]}{K}} = \sqrt{\frac {0,040\mathrm{M}}{2,8\mathrm{M^{-1}}}} = \sqrt{0,01428571\mathrm{M^2}} = \\ &= 0,11952286\mathrm{M} ≈ 0,12\mathrm{M}\end{aligned}\]
