Good comments palgia.
PS. Correct me if i'm wrong, but the standard atmosphere engineers use is 21% oxy, 78% nitrogen and a few other gasses. No moisture? Are they calculating numbers for DRY AIR?
My understanding, after a bit of digging, is that the standard atmosphere is defined using the hyrdostatic constant
g * Mm / R = 34.163 K/km
That would make the molar mass Mm = 28.95 g/mol
I believe that is dry air (approx 78% N2, 21% O2, 1% Ar).