AQA AS chem multiple choice ques

How would you answer this?

I don't really understand molecular ionsassuming that chlorine exists as 2 isotopes, and that hydrogen and carbon exist as 1 isotope each, how many molecular ion peaks will be shown in the mass spectrum of C4H6Cl4?

chlorine has 2 isotopes. the number of molecular ion peaks in the mass spec of Cl2 is:

Chlorine has 3 peaks.

³⁵Cl-³⁵Cl

³⁵Cl-³⁷Cl

³⁷Cl-³⁷Cl

what do you mean by Cl35 - Cl35

Cl has two isotopes. Cl 35 and Cl 37. The weighted mean mass gives 35.5 in the periodic table.

I know that but does that mean if there are 3 isotopes there will be 4 peaks or like chlorine 2 isotopes = 3 peaks?

How would you answer this?

I don't really understand molecular ions

assuming that chlorine exists as 2 isotopes, and that hydrogen and carbon exist as 1 isotope each, how many molecular ion peaks will be shown in the mass spectrum of C4H6Cl4?

chlorine has 2 isotopes. the number of molecular ion peaks in the mass spec of Cl2 is:

Molecular ion is just - it's the whole molecule - no fragmentation, just the loss of an electron.

Chlorine has two isotopes, Cl³⁵ and Cl³⁷; so in a compound containing one chlorine compound there's two molecular ions; M and M+2: one where the chlorine is Cl³⁵ and one where it's Cl³⁷. For dichloro- compounds you've got the M+4 where both chlorines are Cl³⁷ in addition to the M and M+2. Likewise trichloro- compounds have an M+6 peak (all three the heavier isotope), and tetrachloro- compounds will have an M+8 peak in addition to the others. So C₄H₆Cl₄ has five molecular ions. M; M+2; M+4; M+6 and M+8; giving five molecular ion peaks, although the intensities of the higher peaks will be a lot lower due to the relative abundance of Cl³⁵ compared to Cl³⁷.

What about the carbon and hydrogen? I understand what you're saying but what do you mean by M+ and M+2 etc.

You're told to assume that carbon and hydrogen only have 1 isotope; so there's only one potential molecular weight for them.

M is the molecular ion; the mass of the molecule (assuming chlorine is present solely as Cl³⁵) M+2 is that plus 2 mass units - assuming that one of the chlorines is Cl³⁷ since Cl³⁷ is two mass units heavier than Cl³⁵. M+4 is the mass of the molecule plus 4 mass units, assuming 2 chlorines are the heavier isotope. M+6 is mass + 6 mass units for 3 of the heavier chlorine isotope, and so on.

so M+4 is Cl37 and Cl37 ? Back to the original ques, so the compound contains Cl4: M + M+2 and ??? I'm confused :'( I still don't understand

So M+4 for Chlorine would be Cl³⁷ - Cl³⁷ - both the heavier isotope, and the peak for it would at an m/z four more than the Cl³⁵-Cl³⁵ compound.

For your tetrachlorobutane, there's M; M+2; M+4; M+6 and M+8 - all the different potential combinations of Cl³⁵ and Cl³⁷.

please could you write out the combinations like cl35 and cl37 because I don't understand M and M+2. sorry for causing you trouble

Sure:

M - 4 x Cl³⁵
M+2 - 3 x Cl³⁵; 1 x Cl³⁷
M+4 - 2 x Cl³⁵; 2 x Cl³⁷
M+6 - 1 x Cl³⁵; 3 x Cl³⁷
M+8 - 4 x Cl³⁷

So M would be with the chlorines contributing 140 m.u. to the weight of the molecular ion. M+2 has chlorine contributing 142 m.u. thanks to the extra 2 m.u. from the heavier isotope. M+4 has the chlorines at 144 m.u., 2 extra m.u. from each of the chlorine, a total of 4 extra m.u. etc.

Thanks. so what would it be for Cl4 - I might ask my teacher to explain this to me as I don't get molecular ions as a whole

You wouldn't have Cl4, that's not a compound you could form; chlorine gas is diatomic, so Cl2.



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Sorry its C4H6Cl4

Posted from TSR Mobile

Chlorine has two isotopes, Cl³⁵ and Cl³⁷; so in a compound containing one chlorine compound there's two molecular ions; M and M+2: one where the chlorine is Cl³⁵ and one where it's Cl³⁷. For dichloro- compounds you've got the M+4 where both chlorines are Cl³⁷ in addition to the M and M+2. Likewise trichloro- compounds have an M+6 peak (all three the heavier isotope), and tetrachloro- compounds will have an M+8 peak in addition to the others.

So C₄H₆Cl₄ has five molecular ions. M; M+2; M+4; M+6 and M+8; giving five molecular ion peaks, although the intensities of the higher peaks will be a lot lower due to the relative abundance of Cl³⁵ compared to Cl³⁷.

That's tetrachlorobutane, which is the molecule I've been talking about in this thread. It's five peaks as below

How would you explain the original ques to a GCSE student (i think this would make it easier for me to understand). Thanks

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assuming that chlorine exists as 2 isotopes, and that hydrogen and carbon exist as 1 isotope each, how many molecular ion peaks will be shown in the mass spectrum of C4H6Cl4?

chlorine has 2 isotopes. the number of molecular ion peaks in the mass spec of Cl2 is:

The molecular ion is the one at m/z 194 - the molecular weight of C4H6Cl4 if the Cls were of mass 35. You're told that hydrogen and carbon are there as one isotope, so you can discount the effects of different isotopes of those into the peaks; and you're told that there are 2 isotopes of chlorine. So the simplified question you're actually answering is how many different combinations of Cl35 and Cl37 (the two chlorine isotopes) are possible?

Again the molecular ion is the one at the m/z of Cl2 - there's two isotopes of chlorine, in how many ways could they combine in Cl2?