I'm not a huge fan of thermodynamics, I hate physical chemistry.
you could quite easily use pV=nRT, the more accurate equation though would be the Van der Waal equation
(p+(n^2a/v^2)) * (v-nb)=nRT
you can easily find the a and b constants for air as you know the mole fractions, I'll help you out here, as I have the text in front of me with a nice table. a=1.3725 b=0.0372 and you can class air as having a molecular weight of 28.85 at those figures.
You can now plug your figures into the equation.