What proportion of a nitrogen tetroxide and nitrogen dioxide mixture consists of dimers at room temperature and atmospheric pressure?

At ambient (room) temperature and atmospheric pressure, approximately 80 percent of the molecules in a mixture of nitrogen dioxide and nitrogen tetroxide exist as dimers (N₂O₄), while the remaining molecules exist as monomers (NO₂).1

These two forms exist in a state of chemical equilibrium represented by the reversible reaction:

$$2N{O}_2\rightleftharpoons N_2{O}_4$$

The distribution of these molecules is sensitive to changes in temperature and pressure. Because the dissociation of one dimer into two monomers increases the total number of gas molecules, the equilibrium shifts in favor of the dimer under increased pressure and in favor of the monomer under reduced pressure.1