A: The ideal gas law is a very reasonable assumption for most gases you'll see on the PE exam — air, combustion gases, refrigerant vapor in the superheated region. Where it breaks down is near saturation (high pressure, low temperature relative to the critical point), which is why we use steam tables and refrigerant tables rather than the ideal gas law for two-phase or subcooled conditions. For this altitude density problem, air behaves as an ideal gas, so the PV = nRT relationship gives you a clean proportionality between pressure and density.