1. The problem statement, all variables and given/known data A gas obeys the Van Der Waal equation of state. The gas undergoes a free expansion from volume Vi to Vf at a constant temperature T. Find the change in entropy of the gas. 2. Relevant equations du=Tds-Pdv+mdn 3. The attempt at a solution I can solve the problem assuming du= 0 but this shouldn't be right. I can't seem to find out how to find out how much heat enters to keep the gas at a constant temperature. Everything shows that for a free expansion q=0, but here this is not the case.