A tank initially contains a gas mixture with 21 mol % oxygen and 79 mol% nitrogen. Pure nitrogen enters the tank, and a gas mixture of nitrogen and oxygen exits the tank. The molar flow rate of both the inlet and exit streams is 8 mol s^-1.
In addition, use the following data and assumptions
• Assume the tank contents to be well mixed
• Assume ideal gas behavior
• The temperature and pressure inside the tank are held constant
• Molar density of the gas mixture in the tank is constant at 40 mol m^-3
If the volume of the tank is 20 m³ then the time (in seconds) required for oxygen content in the tank to decrease to 1 mol % is