https://www.physicsforums.com/attachment.php?attachmentid=20388&stc=1&d=1252066499 When the partition removed, gas flows into the vacuum until all system has a uniform pressure. Now, is it possible to calculate the work? Let's say gas is not an ideal gas.(Freon 12) We know the volumes of both sides. Also the mass and the initial pressure of gas. I think we can't use W=PdV because pressure is not constant. Is there a way to do it? Edit: By the way the temperature will change but in the end returns to its initial value. So heat transfer will occur through the boundaries.