Which of the following formulas is used to calculate the compressibility factor?

For one mole of a van der Waals gas when $b=0$ and $T=300 \ K$,the $PV$ vs. $1/V$ plot is shown below. The value of the van der Waals constant $a$ (in $\text{atm} \cdot \text{liter}^2 \cdot \text{mol}^{-2}$) is:

If $Z$ is a compressibility factor,the van der Waals equation at low pressure can be written as:

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?

Define the compressibility factor $(Z)$ in terms of the molar volume of a gas.

The van der Waals equation is valid for $......$.