Here is the problem: "A careless experimenter left the valve of a tank of helium slightly open over the weekend. The gas, originally at 200 atm, slowly escaped isothermally at 20 degrees C. What change in entropy per kg of gas occured? " I am given, K, N, and Mass of helium per mole. I am stuck because all I know is: Delta S = N k Ln[Vf/Vi] and P=NkT/V I can't seem to figure out how to make these work though given the problem at hand. How do I find out what N-final and V-final are? That is assuming I even need to find those out to solve this problem. :uhh: Any help is greatly appreciated!