If the compressibility factor $Z < 1$ at $STP$,what will be the molar volume?

The Van der Waals parameters for gases $W$,$X$,$Y$,and $Z$ are given below. Which one of these gases has the highest critical temperature?

$Gas$	$a \ (atm \ L^2 \ mol^{-2})$	$b \ (L \ mol^{-1})$
$W$	$4.0$	$0.027$
$X$	$8.0$	$0.030$
$Y$	$6.0$	$0.032$
$Z$	$12.0$	$0.027$

Which of the curves $(Z \ vs \ p)$ will be followed by a real gas?

Van der Waals equation for a gas is stated as,
$p = \frac{nRT}{V - nb} - a\left( \frac{n}{V} \right)^2$
This equation reduces to the perfect gas equation,$p = \frac{nRT}{V}$,when:

Gases deviate from ideal gas behavior because the molecules $...$

The unit of the van der Waals gas equation parameter $a$ in $(P + \frac{an^{2}}{V^{2}})(V - nb) = nRT$ is :