Derivation of Critical Constants from Van Der Waals Constant

The table indicates the value of van der Waals constant ‘a’ in (dm³)² atm. mol^-2

The gas which can be most easily liquefied is

Four gases P, Q, R and S have almost the same values of 'b' but their 'a' values (a, b are Van der Waals Constants) are in the order Q < R < S < P. At a particular temperature, among the four gases, the most easily liquefiable one is

The units of Van der Waals constants 'b' and 'a' respectively

Derive the values of critical constants in terms of van der Waals constants.