In thermodynamics, the  $P-V$ work done is given by   $w=-\int dV{P}_{ext}.$
For the system undergoing a particular process, the work done is,
$w=-\int dV(\frac{RT}{V-b}-\frac{a}{V^2})$ 
This equation is applicable to a 

$W=-\int P_{ext}dv$

From vander waal equation of state for one mole gas

$$\begin{gathered} (p+\frac{a}{V^2})(V-b)=RT \\ p=(\frac{RT}{(V-b)}-\frac{a}{V^2}) \end{gathered}$$

For reversible process
$$\begin{gathered} p_{ext}=p_{gas} \\ W=-\int (\frac{RT}{(V-b)}-\frac{a}{V^2})dv \end{gathered}$$ 
Process is only not applicable for irreversible process