4. A40ft3 oxygen tank initially contains a 68F and 14.7psia. At a time t=0 a enriched air mixture containing 35%v/vO2 and the balance N2 is added to the tank at 68F and 1.3psig at a rate 60ft3/min and gas is withdrawn simultaneously from the tank at 68F and 14.7psia at a molar flow rate equal to that of the feed gas. a. Calculate the total kg moles of gas (O2+N2) in the tank at any time. b. Let x(t) equal the mole fraction of oxygen in the outlet stream. Write a differential mole balance on oxygen by assuming that the tank contents are perfectly mixed. Convert the balance equation for dx/dt and provide an initial condition. c. Integrate the equation to obtain an expression for x(t). How long will it take for the mole fraction of oxygen in the tank to reach 0.27