The equation of a state of gas is given by $PV = nRT + Pb \rightarrow P (V-b) = nRT$

In an isothermal reaction, $P (V-b) = nRT =$ constant.

Therefore, work done $= \int_{2V}^{V} PdV = \int_{2V}^{V} \large\frac{nRT}{V-b}$ $dV = nRT\log \large\frac{2V-b}{V-b}$