Br question 2.25

[johnwandering]

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Can someone walk me through this problem? It doesnt make a lick of sense to me. The book walks though the solution like its a common sense deduction from the information and graph but it seems like its just obvious memorization.

It says that the "lowest E emission" of the balmer range is 656 nm.
I can eliminate A easily.
But B can be true or Not, because the lyman series just has to be under some value under 656 (considering we are not give a range or relative value whatsoever, the lyman could be any number under 656). For all we know the highest value might be 425.
C can be true or not because the paschen series just has to be over some value above 656. (again considering there is no range or relative value given, it could be any under 656). For all we know the lowest value might be 760.
Again the same thing could be said for D.

Because no relative values are given. None of the choices can be definitively eliminated.


Im pretty sure im missing something. Any help would be much appreciated!!

 

[SaintJude]

More people may be able to help if you post a pic of the question...

 

[MedPR]

I remember doing this question. Pay really close attention to the levels and wavelengths given in that image. Write it out on scratch paper if you have to. Alternatively, post the image and we can help you.

 

[johnwandering]

Sorry i dont have a camera. But it is on page 112 of the br chem 1 book.

But i did write it out and i confirmed that there is no way to prove/disprove that the lyman series are less than 400, paschen series are above 800, and the bracket series is above 800. There is no way to deduce a positive answer without prior knowledge as to the wavelength values/magnitudes.

I am pretty sure its just a flawed question.
Thanks!
=)

 

[MedPR]

You seem to be ok with answer A, so i'll start with B.

All of the info you need is in figure 2.32 on page 111.

B. The lowest energy lyman transition is from n2-n1. This one is more energetic than n3-n2, so you know that the wavelength must be less than 656nm. You can also see, pretty clearly, that n2-n1 is larger than n5-n2, which is 380nm. So n2-n1 must be more energetic than n5-n2, and therefore have wavelength less than 380nm. B is TRUE, therefore incorrect.

C. The lowest energy paschen is n4-n3, which is smaller than n3-n2, so it must have a longer wavelength. So paschen wavelength is longer than 656nm for sure. Just below figure 2-23, it tells you that n4-n2 is 486nm. Since n3-n2 is 656 and n4-n2 is 486, that means that the n4-n2 transition is more energetic than the n3-n2 transition by 170nm. 656+170=826nm. Looking at the size of the energy transitions, you can see that the n4-n3 would be about 826nm. C is TRUE, therefore incorrect.

D. The highest energy bracket is infinity-n4. This is way smaller than n3-n2, so it must be less energetic than n3-n2 and therefore be longer than 656nm. Again, looking at figure 2-32, you can see that it is going to be greater than 800nm because it is smaller than n4-n3, which we've already calculated to be about 826nm. D is FALSE, and therefore correct.

 

[johnwandering]

Thanks for the help!
🙂


But that was pretty much exactly the problem I was having.

The book walks though the solution like its a common sense deduction from the information on the graph but it seems like its just obvious memorization.

I don't know if we have different versions of the book, but there are NO numbers associated with figure 2-32 in my book.
The only way to know that n5-n2 is 380 nm or that n4-n2 is 486nm is to read the paragraph on the bottom of figure 2-33 on page 111 and then memorize it for this problem.

Like I said, it seems that the only way to solve the question is memorizing the numbers.

Perhaps BR had the wavelength numbers associated with figure 2-32 in a previous version and changed it without altering the following question?

 

[ipodtouch]

Yeah, I came across that question before... impossible to solve without knowing those numbers.
1.) The question stem doesn't even reference figure 2-32, which doesn't even have any numbers anyways.
2.) The question doesn't include any other numbers or magnitudes other than the 656 nm.




BR is just telling us to memorize all of those numbers.

 

[MedPR]

Thanks for the help!
🙂


But that was pretty much exactly the problem I was having.


I don't know if we have different versions of the book, but there are NO numbers associated with figure 2-32 in my book.
The only way to know that n5-n2 is 380 nm or that n4-n2 is 486nm is to read the paragraph on the bottom of figure 2-33 on page 111 and then memorize it for this problem.

Like I said, it seems that the only way to solve the question is memorizing the numbers.

Perhaps BR had the wavelength numbers associated with figure 2-32 in a previous version and changed it without altering the following question?

I bought my books new from TBR in December 2011. My book has those numbers in the paragraph below as well. By outside knowledge, I thought you meant information not directly presented by TBR. Even if you didn't use the data presented, you could probably have deduced that answer D was the most probable, and therefore best answer.

I agree, though, that the question was excessive and way harder than something you would see on the MCAT. (Probably)