JEE Advanced 2020 Paper 1

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