tbr gen chem gas question

[capn jazz]

So TBR Chem Chapter 6, Question 88 seems to have an error in either the question or the solution. The question asks about changing the spin rate of a cylinder (with 1 slit so molecules can enter from one end and exit at the other end when that slit has rotated around to allow that to happen) if methane is replaced with NO. Since CH4 is smaller than NO, CH4 has a higher velocity, and so replacing CH4 with NO means that the cylinder has to spin more slowly for the bigger, slower NO molecules to reach the other side at the same time as the slit.

The answer talks about how the cylinder spin rate must be INCREASED to be in phase with the passing METHANE molecules. So did TBR flip this around or am I crazy??

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[matth87]

So TBR Chem Chapter 6, Question 88 seems to have an error in either the question or the solution. The question asks about changing the spin rate of a cylinder (with 1 slit so molecules can enter from one end and exit at the other end when that slit has rotated around to allow that to happen) if methane is replaced with NO. Since CH4 is smaller than NO, CH4 has a higher velocity, and so replacing CH4 with NO means that the cylinder has to spin more slowly for the bigger, slower NO molecules to reach the other side at the same time as the slit.

The answer talks about how the cylinder spin rate must be INCREASED to be in phase with the passing METHANE molecules. So did TBR flip this around or am I crazy??

I thought you replaced methane with NO. How are the methane molecules still present?

 

[Fort]

This was a tough passage.

If you look at the equation v~sqrt(T/m) you'll notice the less massive methane molecules will move faster than the nitrous oxide molecules. If you do a little calculation, you'll find that these molecules will move about 1.61 times faster than oxide at the same temperature. Now the goal is to ensure that these molecules are entering the hole near valve three just as the hole in the cylinder is aligned with that valve (which contains the detector). Well, if the molecules are moving faster than normal, the hole in the cylinder would have to reach valve three in a shorter amount of time because the methane molecules are able to traverse the cylinder faster than the nitrous oxide molecules. The only way to accomplish this is to speed it up by a factor of 1.61 - the same factor with our new system of methane molecules.

I got this question wrong too, so don't feel too bad.

Edit: must be an error in the question then; your reasoning is right. When I read the question I thought methane was replacing nitrous oxide, not the other way around (I think this is what they had in mind when they wrote the question). So you're right, the answer in the back is wrong I think.