Van der Waal's Equation of state for one mole

$\left(\text{P}+\frac{\text{a}}{{\text{V}}^2}\right)\left(\text{V}-\text{b}\right)=\text{RT}$

At low pressure, the characteristic constant (a) becomes significant whereas (b) becomes negligible. So (b) is ignored.

$\left(\text{P}+\frac{\text{a}}{{\text{V}}^2}\right)\left(\text{V}\right)=\text{RT}$

$\text{PV}+\frac{\text{a}}{\text{V}}=\text{RT}$

$\frac{\text{PV}}{\text{RT}}=\frac{\text{RT}}{\text{RT}}-\frac{\text{a}}{\text{VRT}}=1-\frac{\text{a}}{\text{VRT}}$

At low pressure for most gases z < 1.