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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 $20\: m^3$, then the time (in seconds) required for oxygen content in the tank to decrease to $1\:mol\:\%$ is

- $100.45$
- $304.45$
- $3.445$
- $10$