http://en.wikipedia.org/wiki/Brownian_motion

Brownian motion maybe

onereddog - do you have a link for this quote from you?

it quickly separates and rises up. I understand there is virtually no H2 in earth's atmosphere, its so light it freely travels at approx the speed of sound out towards space.

Hi Bilnogatz, re my quote above.. some further background; this thread relates to safe practices for storage & usage of H2. While explaining general properties of H2, a visiting certifier stated how released H2 from a tank will rise very quickly, illustrating the importance of vents at the apex of roofs. He stated “approx the speed of sound” but I’m not sure of this fact(?) Point being it is fast, and hence the question re the scenario of contained air & hydrogen, and why its proportion remains constant.

Yes, Brownian Motion seems to be the key answer; i.e. the kinetic energy of gas molecules overrides the effect of gravity within the cylinder, so the 2% ratio is preserved. I hope I’ve got that right(?)

I presume the size of the cylinder is also a factor. If, hypothetically, it was large enough to have different pressures at its base compared to its top, then there will indeed be a different ratio mix of gas(?)

I reckon this has the makings of a good exam question!

Thanks..