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August 31, 2014

Homework Help: AP Chem

Posted by Belinda on Tuesday, January 23, 2007 at 9:56pm.

When NH3 gas is introduced at one end of a long tube while HCl gas is introduced simultaneously at the other end, a ring of white ammonium chloride is observed to form in the tube after a few minutes. This ring is closer to the HCl end of the tube than the NH3 end. Why? I am completely confused as to why a ring forms and also why is is closer to the HCl end.

The ring forms because
NH3 + HCl ==> NH4Cl and NH4Cl is a white solid. It condenses (at least it aggregates) on the walls of the (glass?) tube. What do you know from the effusion/diffusion law (Graham's Law)?
The NH3 molecules and HCl molecules diffuse through the tube, moving toward each other. Which one moves faster? The smaller one, of course (the one with the lower mass). Which is the lower mass? NH3 has a molar mass of 17 and HCl is about 36.5. So HCl will move slower, NH3 will move faster, so the ring forms closer to the _______ end.

Thanks, that makes sense. But how can HCl and NH3 diffuse through a glass tube?

They don't. I think the problem states that NH3 is placed at the END of the tube and HCl is placed at the other END of the tube. It need not be glass but glass or plastic allows us to see where the white ring forms. Most of us don't have x-ray vision so a metal tube would make it difficult to demonstrate Graham's Law. Anyway, the vapors move through the tube from the ends and they meet, not in the middle but closer to the HCl end because the HCl vapors move slower than the NH3 vapors.

OK, I was just confused at your wording I think.

By the way, by measuring the distances traveled by the NH3 and HCl, we can calculate the ratio of the molar masses.

Which is the theoretical effusion of NH3?
and the the theoretical effusion of HCl?

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