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diffusion of gasses

I have a problem that I am working on, but I don't know the reason why the answer is what it is. Explanation please?
The experimental apparatus represented above is used to demonstrate the rates at which gases diffuse. when the cotton balls are placed in the ens of a tube at the same time, the gases diffuse from each end and meet somewhere in between, where they react to form a white solid. which of the following combinations will produce a white solid closest to the center of the tube?
Answer: HCl and CH3NH2
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2 Answers

Hey Amy -- HCl mw is about 36 ... CH3NH2 mw is about 31
... nearly identical masses meet "in the middle" ... Best wishes, ma'am :)


The other candidate mw pairs are likely a "mismatch" -- one heavy and the other light
where the lighter gas races to meet the heavy gas at the "heavy" end of the tube :)
Hi Amy,
It might help if you describe the apparatus a bit more, also if you could offer the other answer choices?  Also, here's a hint that should help immensely with understanding the idea, it's Graham's Law:
Which says that the relative rates of effusion of two different gases are inversely proportional to the square roots of the gasses' masses.
Rate1/Rate2 = sqrt(M2/M1)
I hope this was helpful!  Do let us know...