What is the mass of oxygen taken out of the cylinder when its initial volume was 30 liters, initial gauge pressure was 15 atm, initial temperature was 27°C, and it later drops to a gauge pressure of 11 atm and a temperature of 17°C? (Given: R = 8.31 J ~mo l ⁻¹ ~ K ⁻¹ , molecular mass of O ₂ = 32 u).