# A Level Chem Mass Spectroscopy help? :((

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Thread starter 9 months ago
#1
How are chlorine molecules 70, 72 and 74 in the ratio 91 on the mass spectrum thing? I'd add a picture of it but it wont let so pls google it and you'll see what I mean
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9 months ago
#2
(Original post by Ho ho cham)
How are chlorine molecules 70, 72 and 74 in the ratio 91 on the mass spectrum thing? I'd add a picture of it but it wont let so pls google it and you'll see what I mean
It's statistics.

If you have 75% chlorine-35 and 25% chlorine-37 there is a 3 in 4 chance of selecting a chlorine-35 atom and a 1 in 4 chance of selecting a chlorine-37 atom.

To make a molecule of Cl2 you have to select two atoms.

If you wish to make a molecule that consists of 35Cl35Cl you have a (3/4 x 3/4) chance of selecting the two chlorine atoms = 9/16

If you wish to make a molecule that consists of 35Cl37Cl you have a (3/4 x 1/4) chance of selecting the two chlorine atoms = 3/16
but for m/z = 72 you could also choose a molecule that consists of 37Cl35Cl you have a (1/4 x 3/4) chance of selecting the two chlorine atoms = 3/16
Hence for m/z = 72 you have a 6/16 chance.

If you wish to make a molecule that consists of 37Cl37Cl you have a (1/4 x 1/4) chance of selecting the two chlorine atoms = 1/16

Hence the probability of getting each m/z value 70, 72 and 74 are in the ratio 91
1
9 months ago
#3
75% of Cl atoms are chlorine-35, and 25% are chlorine 37. In a Cl2 molecule, there're two chlorine atoms, so there are four possibilities:
1 - Atom A is 35Cl and atom B is 35Cl, for a total mass of 70;
2 - Atom A is 35Cl and atom B is 37Cl, for a total mass of 72;
3 - Atom A is 37Cl and atom B is 35Cl, for a total mass of 72 (again);
4 - Atom A is 37Cl and atom B is 37Cl, for a total mass of 74.

The chance of possibility 1 is 3/4 * 3/4, which is 9/16.
The chance of possibility 2 is 3/4 * 1/4, which is 3/16.
The chance of possibility 3 is 1/4 * 3/4, which is also 3/16.
The chance of possibility 4 is 1/4 * 1/4, which is 1/16.

Since (2) and (3) both have a mass of 72, the chance that the mass of the molecule is 72 is 3/16 + 3/16 = 6/16.

So, you have a 9/16 chance of having 70 mass; 6/16 chance of having 72 mass; and 1/16 chance of having 74 mass.
This causes the 91 ratio on a mass spectrum.
1
9 months ago
#4
Please do not make multiple threads on the same topic. It's SPAM.
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Thread starter 8 months ago
#5
(Original post by charco)
It's statistics.

If you have 75% chlorine-35 and 25% chlorine-37 there is a 3 in 4 chance of selecting a chlorine-35 atom and a 1 in 4 chance of selecting a chlorine-37 atom.

To make a molecule of Cl2 you have to select two atoms.

If you wish to make a molecule that consists of 35Cl35Cl you have a (3/4 x 3/4) chance of selecting the two chlorine atoms = 9/16

If you wish to make a molecule that consists of 35Cl37Cl you have a (3/4 x 1/4) chance of selecting the two chlorine atoms = 3/16
but for m/z = 72 you could also choose a molecule that consists of 37Cl35Cl you have a (1/4 x 3/4) chance of selecting the two chlorine atoms = 3/16
Hence for m/z = 72 you have a 6/16 chance.

If you wish to make a molecule that consists of 37Cl37Cl you have a (1/4 x 1/4) chance of selecting the two chlorine atoms = 1/16

Hence the probability of getting each m/z value 70, 72 and 74 are in the ratio 91
ohmygod I only saw this now thank you so much it all makes sensE :'D
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