The van der Waals' equation for a gas is $\left(P + \frac{a}{V^2}\right)(V - b) = nRT$ where $P, V, R, T$ and $n$ represent the pressure, volume, universal gas constant, absolute temperature and number of moles of a gas, respectively. $a$ and $b$ are constants. The ratio $\frac{a}{b}$ will have the following dimensional formula