A 100 ml flask containing Oxygen gas at 1.2 atm and 300 K is connected to a 250 ml flask containing NO at 0.6 atm and 300 K by means of a narrow tube of negligible volume where they combine quantitatively to form NO 2 . Finally NO dimerizes partially to N2O4 and pressure inside the flask measures to be 0.41 atm at same temperature.