The following expression of P-V work done W=−∫PextdV is applicable for reversible isobaric as well as isothermal and adiabatic process. If P/ext is substituted for a Vander Waals equation then it becomes W=−∫(
RT
V−b
−
a
V2
)dV But it is not applicable for irreversible process which are carried out very fast. So work done is calculated assuming final pressure remains constant throughout the process