The pressure of a van der Waals gas is less than the pressure of an ideal gas because of

The relation between pressure exerted by an ideal gas $(P_{ideal})$ and observed pressure $(P_{real})$ is given by the equation
$P_{ideal} = P_{real} + \frac{an^2}{V^2}$
$(i)$ If pressure is taken in $N \ m^{-2}$,number of moles in $mol$,and volume in $m^3$,calculate the unit of $a$.
$(ii)$ What will be the unit of $a$ when pressure is in atmosphere and volume in $dm^3$?

What are the ideal pressure and volume of a real gas?

$A$ van der Waals' gas may behave ideally when

The van der Waals equation reduces to the ideal gas equation under which of the following conditions?

If the compressibility factor $(Z)$ is greater than unity,then which of the following is true for the molar volume $(V_m)$ of a gas at $NTP$?