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# Need help with mass spectrum ocr watch

1. Attachment 527459527463 Hi, can anyone please explain to me how to do part c? and c ii) and iii) from the second attachment? I have done the a and b) but I dont understand why 35Cl2 + would give a m/z value of 70. 35*2-1=69, why would it be 70? I don't really understand this, thanks in advance.
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2. Is that OCR B (Salters)?
3. (Original post by Pigster)
Is that OCR B (Salters)?
I am not sure but it's the new syllabus . Do you have any idea how to do this question ? Thanks
4. (Original post by coconut64)
Attachment 527459527463 Hi, can anyone please explain to me how to do part c? and c ii) and iii) from the second attachment? I have done the a and b) but I dont understand why 35Cl2 + would give a m/z value of 70. 35*2-1=69, why would it be 70? I don't really understand this, thanks in advance.
For the chlorine molecule there are three possible molecular ions:

[35Cl-35Cl]+
[35Cl-37Cl]+
[37Cl-37Cl]+

with mass/charge ratios of 70, 72 and 74 respectively.

There are two possible ways to obtain the [35Cl-37Cl]+ ion.

... and when you consider the relative probability of the two isotopes, (35=3/4 and 37 = 1/4) the ratio of the molecular ions comes out to 9 : 6 : 1, which is what is observed in terms of the intensity of the signals.

Relative probability of [35Cl-35Cl]+ = 3/4 x 3/4 = 9/16

Relative probability of [35Cl-37Cl]+ = (1/4 x 3/4)x 2 = 6/16

Relative probability of [37Cl-37Cl]+ = 1/4 x 1/4 = 1/16

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