tc and pc of gas are 400k and 41 atm respectively then vc is

Critical temperature is the temperature above which a gas cannot be liquified. It is denoted as T_c.
p_c is the Critical pressure, V_c is the critical molar volume
R= 0.082 L atm K^-1 mol^-1

V_c = 3b  (from Ideal gas law)
​and b = $\frac{{\mathrm{RT}}_{\mathrm{c}}}{8{\mathrm{p}}_{\mathrm{c}}}=\frac{0.082 \mathrm{L} \mathrm{atm} {\mathrm{K}}^{-1} {\mathrm{mol}}^{-1}\times 400\mathrm{K}}{8\times 41\mathrm{atm}}=0.1 {\mathrm{Lmol}}^{-1}$
V_c = 3b = 0.3 Lmol^-1