# Need HELP with drawing the mass spectra graph!!!

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#1
I drew the lines for 79 and 81 only but I don't know how tall they should be or their % abundance!

0
5 years ago
#2
the % abundance for each isotope will be 50% as half is one isotope and the other half is the other isotope
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#3
(Original post by fgardhouse)
the % abundance for each isotope will be 50% as half is one isotope and the other half is the other isotope
Is it always the same case for every isotope or just this one?
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5 years ago
#4
it isn't always the same but in the question it says that halve the sample is 79-Br and the other halve is 81-Br
1
5 years ago
#5
Is it always the same case for every isotope or just this one?
Just this one.

The height of the signal is directly proportional to the natural abundance of the isotope ...
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#6
(Original post by charco)
Just this one.

The height of the signal is directly proportional to the natural abundance of the isotope ...
I see, so this is the markscheme. How do I know the heights of 158, 160 and 162?!
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5 years ago
#7
I see, so this is the markscheme. How do I know the heights of 158, 160 and 162?!
This is statistics.

If you have two isotopes with equal probability and diatomic molecules then there are three possible masses for the molecules, 158, 160 and 162:

But there is twice as much chance (probability) of the molecule being 160

79Br79Br = 158
79Br81Br = 160
81Br79Br = 160
81Br81Br = 162

Hence the signal for 160 is twice as high ...
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#8
(Original post by charco)
This is statistics.

If you have two isotopes with equal probability and diatomic molecules then there are three possible masses for the molecules, 158, 160 and 162:

But there is twice as much chance (probability) of the molecule being 160

79Br79Br = 158
79Br81Br = 160
81Br79Br = 160
81Br81Br = 162

Hence the signal for 160 is twice as high ...
So if the question lets say was about Carbon for instance, do you work it out in the same way, so

C12 x C12
C13 x C13
C14 x C14

C12 x C13
C13 x C14
C12 x C14

C12 x C13 x C14
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5 years ago
#9
So if the question lets say was about Carbon for instance, do you work it out in the same way, so

C12 x C12
C13 x C13
C14 x C14

C12 x C13
C13 x C14
C12 x C14

C12 x C13 x C14
Carbon cannot make diatomic molecules ..

It gets more complicated when the natural abundance of each isotope is not 50%.

In chlorine for example the ratio of heights for Cl2 is 91, for 70, 72 and 74 respectively
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#10
(Original post by charco)
Carbon cannot make diatomic molecules ..

It gets more complicated when the natural abundance of each isotope is not 50%.

In chlorine for example the ratio of heights for Cl2 is 91, for 70, 72 and 74 respectively
How did you get the ratio?!
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5 years ago
#11
How did you get the ratio?!
It's statistical probability.

The natural abundances are 75% 35Cl and 25% 37Cl

Hence the probability of a molecule containing two 35Cl atoms is 3/4 x 3/4 = 9/16

the probability of a molecule containing two 37Cl atoms is 1/4 x 1/4 = 1/16

Hence the probability of a molecule containing one 35Cl atom and one 37Cl atom is 16/16 - (9/16 + 1/16) = 6/16

Hence the ratio is 19 with respect to m/z 7470
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