When does the graph of $p \propto \frac{1}{V}$ not result in a straight line?

Van der Waal's equation $\left[ p + \frac{a}{V^2} \right] (V - b) = nRT$ is applicable for

Explain the compressibility factor $(Z)$.

Consider the van der Waals constants,$a$ and $b$,for the following gases:

Gas	$Ar$	$Ne$	$Kr$	$Xe$
$a / (atm \ dm^6 \ mol^{-2})$	$1.3$	$0.2$	$5.1$	$4.1$
$b / (10^{-2} \ dm^3 \ mol^{-1})$	$3.2$	$1.7$	$1.0$	$5.0$

Which gas is expected to have the highest critical temperature?

At a temperature of $27\,^{\circ}C$,if the volume of $2\, \text{moles}$ of $NH_3$ gas is $5\, L$ according to the van der Waals equation,what will be its pressure in $atm$? ($a = 4.17$,$b = 0.03711$)

What is the difference in the pressure-volume $(P-V)$ graph (at constant $T$) for a real gas and an ideal gas?