The Vander Waal's constant 'a' for the gases O2,N2,NH3 and CH4 are 1.3, 1.390, 4.170 and 2.253 L2 atm mol−2respectively. The gas which can be most easily liquefied is
Higher the value of ‘a’ more will be the Vander Waal’s force of attraction between the molecules and hence more easily the gas can be liquefied.