A comment below What exactly are "white fumes" and why does holding a bottle of ammonia (conc) next to a bottle of HCl (conc) make them? links to The Royal Society Of Chemistry video Diffusion in action | The reaction of ammonia and hydrogen chloride demonstration
Declan Fleming revisits the classic diffusion demo of the reaction of ammonia with hydrogen chloride – providing some great tips for doing it even better.
Download the instructions: https://rsc.li/3xsjpxS
As Graham's law doesn't apply to this situation I tend to leave things here...
Although the presence of air molecules makes Graham's law inapplicable...
and the instructions linked in the video's description includes the following:
However, this method neglects the fact that the assumptions behind Graham’s law cannot apply. The different sizes of the molecules and their collision cross sections with other molecules come into play. In other words, the rate of diffusion depends not only on the molecules of HCl and NH3, but also on the properties of the (mainly) nitrogen and oxygen molecules into which they are diffusing. As such, the observed ratio may be slightly less than that predicted in the simplistic model above – the demo is perhaps best kept as a qualitative one at this level.
The ratio of the distances from the boundary where smoke particles are produced and the two sources is 15.5 cm / 8 cm ~ 1.94 and the square root of the mass ratio (if it did apply, which it does not) is (36.46 / 17.03)1/2 ~ 1.46.
Both gasses are diffusing in air, which is not at all the same as effusing from an aperture smaller than the mean free path (of order 10 microns). So I understand why Graham's law does not apply here. So I'd like to ask:
Question: If Graham's law doesn't apply to the ammonia and hydrogen chloride diffusion in a glass tube demonstration, is there a law that does, or at least gets close?
Annotated screenshot from The Royal Society Of Chemistry video "Diffusion in action | The reaction of ammonia and hydrogen chloride demonstration" https://youtu.be/SWByFMo32Qg