# Mass spectrometryWatch

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#1
Q. A hydrocarbon has a molecular ion peak at m/e ratio of 84 (relative abundance of 62%) and an [M+1] peak with a relative abundance of 4.1%.
a. How many carbon atoms are there in the hydrocarbon?
b. What is it's molecular formula?
c. The hydrocarbon does not decolourize bromine water. Name the hydrocarbon

Thank you
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3 months ago
#2
Well I got somewhat of an idea.
Firstly, I would divide through 84 by 12 then round down. We know it can't be exact as there needs to be H involved. I got C7H14. But molecular in peak would be 86. So I'm not 100% sure.
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3 months ago
#3
It works if it does decolourise bromine as it would be C6H12 with molecular ion peak 84
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#4
(Original post by JamesAndrews1)
Well I got somewhat of an idea.
Firstly, I would divide through 84 by 12 then round down. We know it can't be exact as there needs to be H involved. I got C7H14. But molecular in peak would be 86. So I'm not 100% sure.
we can find no. Of carbon atoms by the formula
n=100/1.1 *abundance of M+1 peak/abundance of M peak..
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#5
(Original post by JamesAndrews1)
It works if it does decolourise bromine as it would be C6H12 with molecular ion peak 84
Here it does not decolourize so it's an alkane ... But just assume that if the c part of question wasn't there than how could I find the answer for b part?
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3 months ago
#6
Not entirely sure. The way I approached it yields C6H14 but that has M as 86 gmol-1
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3 months ago
#7
You said molecular ion peak was at 84, right?
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3 months ago
#8
m/e = 84 so the hydrocarbon must be C6H12
It does not decolorise bromine water, so it has no double bonds.
But it does have a double bond equivalent, so it must be cyclic.
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3 months ago
#9
0
3 months ago
#10
(Original post by JamesAndrews1)
... or methylcyclopentane
... or dimethylcyclobutane (cis and trans, 1,2 and 1,3)
... or ethylcyclobutane
... or trimethylcyclopropane (cis and trans)
... or ethylmethylcyclopropane (cis and trans)
... or propylcyclopropane

12 possible isomers (I think)
Last edited by charco; 3 months ago
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#11
(Original post by charco)
... or methylcyclopentane
... or dimethylcyclobutane (cis and trans, 1,2 and 1,3)
... or ethylcyclobutane
... or trimethylcyclopropane
... or ethylmethylcyclopropane
... or propylcyclopropane

10 isomers
Oh thanks a lot
0
3 months ago
#12
(Original post by charco)
... or methylcyclopentane
... or dimethylcyclobutane (cis and trans, 1,2 and 1,3)
... or ethylcyclobutane
... or trimethylcyclopropane (cis and trans)
... or ethylmethylcyclopropane (cis and trans)
... or propylcyclopropane
12 possible isomers (I think)
Yh, I was just being lazy. I didn't know that cyclic alkanes could exhibit cis and trans isomerism. Thanks.
Last edited by JamesAndrews1; 3 months ago
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