1. The problem statement, all variables and given/known data A well insulated container consists of two equal volumes separated by a partition - one half an ideal gas while the other is a vacuum. The partition is removed, and the gas expands. What is the entropy change per mole? 2. Relevant equations dS = dQrev/T S/R = Cp/R dT/T - ln(P/P0) 3. The attempt at a solution As the process is irreversible, the entropy change should be non-zero even though there is no change in heat. However, after I perform the calculation with the aforementioned equation, I got zero as my final answer. I'm extremely confused about my result because I can't find any mistake in my calculation, while I do believe that the change should be non-zero. Any help would be highly appreciated!