Calculations using the ideal gas law will not work. I mean...you'll get a number, but it'll be wrong. You have been warned!
ACKCHYUALLY... For low pressures (<5 psia or so) the ideal gas equation describes steam very well, with a gas constant of 85.78 ft-lbf/(lbm°R) and a constant specific heat of 0.445 Btu/(lbm°R). Modeling steam as an ideal gas is the foundation of the equations used to draw the psychrometric chart. Moist air is modeled as a mixture of two ideal gases: water vapor and dry air. This works because in the typical HVAC application, the partial pressure of water vapor is very low.
HOWEVER: Having said all that, we're with you and discourage trying to solve any problems using the ideal gas equations instead of the tables. The region of the phase diagram in which the ideal gas equation is accurate is also covered by the tables, so there is no need to mess with state equations at all, be it ideal gas, Van derWaals, BWR, etc.
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