/sci/ - Science & Math

>>986114

not this one again... jesus fuck

>>986114

I should probably also mention the compound in question is:

COCl2

>>986112

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>>986114

I tried OP... but I have no idea where to start.

I get 51.04% but that can't be right. The most massive chlorine isotope is 37Cl, and it's the least abundant of Cl isotopes. Therefore the percentage would have to be below 50%... not sure where go to from there.

>>986114

>he can't do 1st year chemistry

>>986255

Yep. Perhaps you'd have the common courtesy to help out since you're so knowledgeable.

* 12C 98.9% (12.000000 amu)
* 13C 1.1% (13.003355 amu)
* 16O 99.76% (15.994915 amu)

Atomic mass of 37Cl isotope = 36.9659 amu
COCl2

Find approx % abundance of most massive isotope of chlorine
present

Mass of Chlorine / Mass of Substance
36.9659 / (12*0.989+13.003355*0.011+36.9659*2)

43.01% of the compound is chlorine

* 12C 98.9% (12.000000 amu)
* 13C 1.1% (13.003355 amu)
* 16O 99.76% (15.994915 amu)

Atomic mass of 37Cl isotope = 36.9659 amu
COCl2

Find approx % abundance of most massive isotope of chlorine
present

Mass of Chlorine / Mass of Substance
2*36.9659 / (12*0.989+13.003355*0.011+36.9659*2)

86.02% of the compound is chlorine

>>986288

Asks for the abundance of the most massive isotope of chlorine (which I believe to be 37Cl), not he abundance of Cl all together.

are you sure you've provided all the information in the problem?

>>986314

I'll copy paste everything to make sure:

In a mass spectrometer, molecules can be ionized into singly positively charged molecular ions which are then separated according to their masses and counted. The resulting spectrum is essentially a histogram showning the relative numerical abundances of ions of different masses.

Phosgene, a highly toxic, gasesous, industrial chemical (used somewhat unsuccessfully as a chemical warfare agent in WWI) has the molecular formula COCl2.
A portion of the mass spectrum of phosgene is represented below.

Given the following natural isotopic abundances (and atomic masses):

* 12C 98.9% (12.000000 amu)
* 13C 1.1% (13.003355 amu)
* 16O 99.76% (15.994915 amu)

Enter the atomic mass of the least abundant isotope of chlorine present in the sample (in amu,to at least 4 decimal places). 36.9659 amu

Enter the approximate % abundance of the most massive isotope of chlorine present in the sample (try to get it within 1%).

From the picture it looks like about 24%.

>>986345

An estimate won't exactly suffice.

>>986306
Oh, ok. So assuming you're allowed to look it up in a chart, then you would find out that Chlorine is grouped like this:

75.77% is Cl-35 (34.9688) and 24.23% is Cl-37 (36.96590)
So all we have to do is modify the origonal answer a bit

Mass of Chlorine 37 / Mass of Substance
2*36.9659*.2423 / (12*0.989+13.003355*0.011+15.994915*.9976+2*36.9659*.2423+2*34.9688*.7577)

18.11% of the compound is chlorine-37

I think you need to estimate the relative abundance of all the three variants, which correspond to:
C12 O16 Cl35 Cl35 -- ~0.57
C12 O16 Cl37 Cl35 -- ~0.37
C12 O16 Cl37 Cl37 -- ~0.06
other: <0.01
now just set relative abundance of 37Cl to x
x~0.245 from the last one

and a wiki search shows it's about .242

>>986376

Cl-35 is 35.45 amu though... right?

>>986387
no. 35Cl has about 35 and 37Cl has about 37. they average to about 35.45.

>>986376

18.11% is incorrect by the way. The answer can be submitted online to check if it's right or wrong, but it won't give you the correct answer until you've got it wrong 5 times.

>>986469
Submitted where?

>>986480

Online homework for my university class. If I get it wrong 5 times, I lose a mark. Shits depressing - first question I've had problems with since I started.

>>986496
have you tried >>986384 yet?
try it. I think it's correct. don't overthink the problem.

>>986496
I think the fact that this is a university homework thread makes it so much worse. You should contact us when you're having problems with your job too.

>>986384
I'm in agreement with this the answer looks to be about right from the graph

>>986384
>>986512

Can you link the wiki?

>>986538
http://en.wikipedia.org/wiki/Chlorine#Isotopes

>>986384
>>986549

Hah, funny enough is it's correct. Perhaps you'd care to explain how I can get the answer based on what I was given in the question though.

>>986570
this mass spec corresponds to the substance COCl2 with different isotope compositions.

already given is the fact that both carbon and oxygen has a major (>.99) isotope so anything with a major peak is gonna have those.

chlorine has two isotopes and thus there are three possible arrangements of the compound (with two Cl atoms).

therefore they largest peak is gonna correspond with the combination of both the major isotopes (in this case you can see it has the smallest mass thus the major isotope is 35Cl)

anyway the important things is the peak with the largest mass correspond with the compound having both the heavier isotopes and thus its abundance is x^2 where x is the abundance of the heavier isotope.

>>986110

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>>986602

So essentially:

Lowest spike = C, O, 37-Cl, 37-Cl
Middle spike = C, O, 37-Cl, 35-Cl
Highest spike = C, O, 35-Cl, 35-Cl

Where exactly do we go from there in terms of determining the answer?

>>986676
Well, then you just do the math.

Looks like around 57% is Cl-35 Cl-35, so 57% has to be Cl-35.
~36% is Cl-35 Cl-37 which gives you +18% Cl-35 and +18% Cl-37, so now you have a total of 75% Cl-35 and 18% Cl-37. Then the rest has to go into Cl-37 so you end up with something like 75% Cl-35 and 25% Cl-37.