Perfect Ideal Gas Modelization of Air I'm solving a Thermodynamic problem. I have Air at temperature of 293.15 K, and pressure of 20 MPa. I would understand if is possible to consider this an Ideal Gas or not (and if is possible which are the limit of pressure and temperature with i can still consider air as an Ideal Gas).
Thankyou very much.
Samuele
 A: I tried a quick Google but couldn't find any data on this. However you can use the Van der Waals equation of state to get some idea how big the deviations are going to be. The ideal gas law (for one mole) is:
$$ PV = RT $$
and feeding in $P = 20\text{MPa}$ and $T = 293\text{K}$ I get $V = 1.22 \times 10^{-4}m^3$. The Van der Waals equation of state is (again for one mole):
$$ \left( P + \frac{a}{V^2}\right)\left(V - b\right) = RT $$
and from this web site I found values for air of $a = 0.1358\text{Jm}^3\text{mole}^{-2}$ and $b = 3.64 \times 10^{-5}\text{m}^3\text{mole}^{-1}$. If we now use these values and the calculated value for $V$ in the Van der Waals equation we get:
$$ \left( 2\times 10^7 + 9.2\times 10^6\right)\left(1.22\times 10^{-4} - 3.64 \times 10^{-5}\right) = RT $$
So the non-ideal terms are large compared to the pressure and volume. I think it's safe to say air is highly non-ideal under these circumstances. In fact with such large corrections even the VdW equation may well be outside its region of validity.
