Isobaric Process

For a process carried out at constant pressure, the work done by the gas simplifies to $W=P\Delta V$, because pressure can be pulled outside the integral $\int PdV$. The volume change must be expressed in SI units: $\Delta V=5.0-2.0=3.0\text{ L}=3.0\times 10^{-3}{\text{ m}}^3$. Multiplying gives $W=(1.0\times 10^5)(3.0\times 10^{-3})=300\text{ J}$. The value 150 J arises from halving the volume change without justification. The value 500 J wrongly uses the final volume of 5 litres instead of the difference. The value 30 J results from forgetting to convert litres to cubic metres, dropping a factor of ten. This constant-pressure expansion is also called isobaric, and on a pressure-volume diagram its work equals the rectangular area beneath the horizontal line joining the initial and final states. As a plausibility check, the gas expands so it does positive work on the surroundings, and the magnitude of a few hundred joules is reasonable for an everyday atmospheric expansion of three litres.