The figure represents $PV$ vs $P$ relation for $CO$, $CH_4$, $H_2$ and $He$ gases under identical conditions. Which curve, shown in the figure, represents $He$ gas?

Gas deviates from ideal gas nature because molecules

Assertion : Compressibility factor for hydrogen varies with pressure with positive slope at all pressures.
Reason : Even at low pressure,repulsive forces dominate hydrogen gas.

For a gas at $STP$,the compressibility factor is greater than $1$. What will be the value of its molar volume $(V_m)$?

$A$ gas has a compressibility factor of $0.5$ and a molar volume of $0.4 \ dm^3 \ mol^{-1}$ at a temperature of $800 \ K$ and pressure $x \ atm$. If it shows ideal gas behaviour at the same temperature and pressure,the molar volume will be $y \ dm^3 \ mol^{-1}$. The value of $x / y$ is . . . . . .
[Use: Gas constant,$R = 8 \times 10^{-2} \ L \ atm \ K^{-1} \ mol^{-1}$ ]

Van der Waals constants '$a$' and '$b$' are related with..... respectively.