States of Matter (Gaseous and Liquid States)

The value of a is a measure of the magnitude of the attractive forces between the molecules of the gas. Greater the value of ‌′a′, larger is the attractive inter-molecular force between the gas molecules.
The value of b related to the effective size of the gas molecules. It is also termed as excluded volume.
The gases with higher value of a and lower value of b are more liquefiable, hence for C12′′a′′′ should be greater than for C2H6 but for it b should be less than for C2H6.