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

The units of constant $a$ in the Van der Waals equation are:

Which statement is correct regarding the equation $Z = PV/RT$?

The compressibility factor for a van der Waal gas at high pressure is

Pressure exerted by $1 \, mole$ of methane in a $0.25 \, L$ container at $300 \, K$ using van der Waals equation is ............... $atm$ (given $a = 2.253 \, atm \, L^2 \, mol^{-2}, b = 0.0428 \, L \, mol^{-1}$):

At high temperature and low pressure,the Vander Waal's equation is reduced to