The dimensional formula of constant ‘a’ in vander-Waal's gas equation
$\left(\mathrm{P}+\frac{\mathrm{a}}{{\mathrm{V}}^2}\right)(\mathrm{V}-\mathrm{b})=\mathrm{RT}$ is 

Here, the dimensions of (a/V²) will be the same as those of P (homogeneity principle). So
$\begin{array}{l}\mathrm{a}=\left[\mathrm{P}\times {\mathrm{V}}^2\right]=\left[\frac{{\mathrm{MLT}}^{-2}}{{\mathrm{L}}^2}\times {\mathrm{L}}^6\right]\\ =\left[{\mathrm{ML}}^5{\mathrm{T}}^{-2}\right]\end{array}$