Where:

- is the mole fraction of O2
- is the moles of oxygen
- is the number of moles in total

Since we now two of the above we can solve for which is the total moles of air

We can then put this into the ideal gas equation:

edit:

I should point out that I change P in atm to Pa so I could use the SI value of R with which I am more familiar. It does not affect the final answer and 1atm = 101325Pa so no rounding off occured