The unit of the van der Waals gas equation parameter ' a ' in (P +...

Pressure correction $\Delta P = \dfrac{{an}^{2}}{{V}^{2}}$

$a = \dfrac{\Delta P \times {V}^{2}}{{n}^{2}}\dfrac{\left(atm ( Lit{)}^{2}\right)}{( \text{mole}{)}^{2}} = \dfrac{atm {\left({dm}^{3}\right)}^{2}}{( \text{mole}{)}^{2}}$

$a = atm \cdot {dm}^{6} \cdot {mol}^{- 2}$