4 moles of an ideal gas at 27°C are allowed to expand D isothermally from an initial pressure of 12.0 atm to a final pressure of 6.0 atm against a constant external pressure of 1.0 atm. Thus, work done will be

Gas is being expanded against external pressure. Thus, work is done by the system, work is negative.

$\begin{array}{l}\mathrm{W}=-{\mathrm{p}}_{\mathrm{ext}}\left[{\mathrm{V}}_2-{\mathrm{V}}_1\right]\\ =-p_{\text{ext }}\left[\frac{nRT}{p_2}-\frac{nRT}{p_1}\right]=-nRT{p}_{\text{ext }}\left[\frac{1}{p_2}-\frac{1}{p_1}\right]\\ =-4\times 0.0821\mathrm{Latm}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-1}\times 1\left[\frac{1}{6}-\frac{1}{12}\right]\times 300\mathrm{K}\\ =-8.21\mathrm{L}\text{ atm }=\frac{8.21\mathrm{Latm}\times 8.314\mathrm{J}}{0.0821\mathrm{L}\text{ atm }}=-831.4\mathrm{J}\end{array}$