A container of volume 50 cm³ is connected with a short tube fitted with a valve to a 100 cm³  container that is evacuated using a vacuum pump. The 50 cm³ container encloses 1 mole of oxygen at 290 K. Once the valve is open, the oxygen undergoes a free and isothermal expansion and fills both containers. What is the entropy change (ΔS) during this free expansion?