The gas with the highest critical temperature is

The compressibility factor $(Z)$ of a gas at critical state is $(T_c = \frac{8 a}{27 R b}; p_c = \frac{a}{27 b^2}; V_c = 3 b)$.

What is the Andrews isotherm graph used for?

Distinguish between vapor and gas.

Four gas cylinders containing $He$,$N_2$,$CO_2$ and $NH_3$ gases separately are gradually cooled from a temperature of $500 \ K$. Which gas will liquefy first?
(Given $T_C$ in $K - He: 5.3, N_2: 126, CO_2: 304.1$ and $NH_3: 405.5$)

The temperature below which a gas cannot exist is called ...............