Air expands from 5 litres to 10 litres at 2 atm pressure. External workdone is

$\begin{array}{l}{\mathrm{V}}_1=5\mathrm{lts}=5\times {10}^{-3}{\mathrm{m}}^3;{\mathrm{V}}_2=10\mathrm{lts}\\ 10\times {10}^{-3}{\mathrm{m}}^3\\ \begin{array}{rl}\mathrm{P}& =2\mathrm{atm}=2\times 1.01\times {10}^5\mathrm{pa}\approx 2\times {10}^5\mathrm{p}.\mathrm{a}\\ \mathrm{dW}& =\mathrm{Pdv}=\mathrm{p}\left({\mathrm{v}}_2-{\mathrm{v}}_1\right)\\ & =2\times {10}^5\left(10\times {10}^{-3}-5\times {10}^{-3}\right)\\ & =2\times {10}^5\times 5\times {10}^{-3}=1000\mathrm{J}\end{array}\end{array}$