# A2 chemistry question relating to mass spectrometry.

Cant quite get my head around this one :

The molecular formula of TCDD is C12H4O2Cl4 Chlorine exists as two isotopes 35Cl (75%) and 37Cl (25%). Deduce the number of molecular ion peaks in the mass spectrum of TCDD and
calculate the m/z value of the most abundant molecular ion peak.

Number of molecular ion peaks ............

m/z value of the most abundant molecular ion peak ..................

Any help with explanations as of how to deduce the correct answers for each part will be appreciated!
(edited 12 years ago)
Original post by kontemptXD
Cant quite get my head around this one :

The molecular formula of TCDD is C12H4O2Cl4 Chlorine exists as two isotopes 35Cl (75%) and 37Cl (25%). Deduce the number of molecular ion peaks in the mass spectrum of TCDD and
calculate the m/z value of the most abundant molecular ion peak.

Number of molecular ion peaks ............

m/z value of the most abundant molecular ion peak ..................

Any help with explanations as of how to deduce the correct answers for each part will be appreciated!

Well there are four chlorine atoms and each of them may be either 35Cl or 37Cl ...

... how many possible combinations are there?
Original post by kontemptXD
Cant quite get my head around this one :

The molecular formula of TCDD is C12H4O2Cl4 Chlorine exists as two isotopes 35Cl (75%) and 37Cl (25%). Deduce the number of molecular ion peaks in the mass spectrum of TCDD and
calculate the m/z value of the most abundant molecular ion peak.

Number of molecular ion peaks ............

m/z value of the most abundant molecular ion peak ..................

Any help with explanations as of how to deduce the correct answers for each part will be appreciated!

The second part of the question requires yo to use probabilities considering that the probability of a chlorine atom being 35Cl is 3/4, while the probability of a chlorine atom being 37Cl is only 1/4.

For example:

to get all four chlorine atoms as 35Cl has a probability of 3/4 * 3/4 * 3/4 * 3/4 = 81/256

The relative probability gives you the relative intensity of the lines.
Thanks, I understand the first part now i.e you could have all 35 or all 37 or 3(35's) and 1(37) etc... totalling to 5 different combinations.

I still don't quite understand part two, do I assume all the chlorine atoms to be of mass 35 and work out the Mr? if so why?
Original post by kontemptXD
Thanks, I understand the first part now i.e you could have all 35 or all 37 or 3(35's) and 1(37) etc... totalling to 5 different combinations.

I still don't quite understand part two, do I assume all the chlorine atoms to be of mass 35 and work out the Mr? if so why?

No. The example I gave you was for all 35s.

You have identified the different possibilities (5), now you have to use the information I gave you to work out the probability of each of the possibilities identified.

The probability of all four chlorines being 35 is 81/256
The probability of all four being 37 is 1/256
The probabilility of there being 1 * 35 and 3 *37 is 3/256 ... BUT there are four ways that this can occur = 12/256

See if you can work out the other two..
(edited 12 years ago)