Now, consider the different chemical mixtures of powder.
I think you are making the problem more complicated than it really is. Let me try my hand at simplifying:
For any mixture of gasses, P1*V1/T1 = P2*V2/T2. Temperature has to be in Kelvins. This applies to the mix of CO2, N2, H2, etc. that is produced by propellants as well as it applies to a pure, ideal gas like Argon.
Now if you want to calculate pressure and temperature from volume, number of moles of gas, etc., the problem is as complicated as you think it is. But as long as you stick to the same mixture of gasses, you can fiddle with temperature, pressure, and volume using the formulas above. Fortunately, that's often enough.
