Just to quantify things, the molecular weights of nitrogen, oxygen, of the blend we call "air," carbon monoxide, and pure propane are:
N2 28.014
O2 31.999
air 28.951
CO 28.010
C3H8 44.097
So, CO is almost exactly the same weight as nitrogen, and one can conclude that molecular diffusion keeps the molecules well mixed, without stratification. Propane is sufficiently heavier that it can stratify, although given time it will diffuse into the bulk air.
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