The Vander Waal's equation for a gas is (P + a/v2)(V - b) = nRT. The ratio b/a will have the following dimensional formula.

M-1L-2T2 M-1L-1T-1 ML2T2 MLT2

M-1L-2T2

Dimension of (b) = L3 Dimension of (a) = ML5T-2 ∴ Dimension of $\left(\frac{b}{a}\right) = \frac{L^3}{ML^5T^{-2}}$ = M-1L-2T2