.T_c​= Temperature at and above which the vapour state can not be further liquified. 

P_c​= Pressure of gas in its critical state

V_c​= Volume of gas in its critical state.

 relation with the van der waals constants -

T_c​=8a / 27Rb

 So a = 27 T_c ​Rb / 8

V_c​ = 3b So b = V_c​/3

P_c​ = a / 27 b² ....(1)

put the value of a and b in equation (1) 
 

Pc​=27Tc​.R / 8×27×b​

=27 Tc​.R / 8×27×Vc​/3​

${\mathrm{P}}_{\mathrm{C}}{\mathrm{V}}_{\mathrm{C}}=\frac{3}{8}{\mathrm{RT}}_{\mathrm{C}}$