Question: 2 moles of a monatomic ideal gas at an initial pressure of 20 atm and a temperature of 450K undergo a rapid (adiabatic) free expansion from a small vessel into a vessel of 40 times greater volume. Find i) the change in temperature and ii) the increase in entropy The first part I know how to do. But for the second part the dQ is 0 for the adiabatic expansion. How am I going to find dS. I know it's not zero. I know adiabatic is not a reversible process. But I don't understand the approach to solve the irreversible problem. Somebody please help.