I'll have a go
For a dry ideal gas Volume/Temperature is a constant provided that Pressure stays the same (Charles' Law).
Pressure * Volume is a constant provided that Temperature stays the same (Boyle's Law).
Combine the two and for a dry ideal gas PV/T is a constant. So if you have two states denoted y suscrits 1 & 2 you can write P1V1/T1 = P2V2/T2. Teerature has to easured (y keyoard can't sell) in asolute units When it coes to wet air it gets a little awkward I can't write now the keys are screwed!