The equilibrium ${\mathrm{N}}_{2\left(\mathrm{g}\right)}+{\mathrm{O}}_{2\left(\mathrm{g}\right)}\to 2{\mathrm{NO}}_{\left(\mathrm{g}\right)}$ established in a reaction vessel of 2.5 L capacity.

The amount of ${\mathrm{N}}_2 \mathrm{and} {\mathrm{O}}_2$ taken at the start were respectively 2 moles and 4 moles. Half a mole of nitrogen has been used up at equilibrium. The molar concentration of nitric oxide is