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molecular ion peaks?

http://filestore.aqa.org.uk/subjects/AQA-CHEM4-W-QP-JUN10.PDF

4b)
the markscheme: 5 Allow 320(.0) or 322(.0)

i don't get it..
could someone explain?please

Reply 1

EierVonSatan

You have two isotopes of chlorine: mass numbers 35 and 37. The difference is between them is 2.

Reply 2

Petulia

Cl-35 is more abundant, so this is the value you use.
Then, you just calculate the Mr of the whole molecule: C12H4O2Cl4
C: 12 x 12
H: 4 x 1
O: 16 x 2 altogether = 182
182 + Cl: 35 x 4 = 322

Reply 3

pureandmodest

Original post by EierVonSatan

You have two isotopes of chlorine: mass numbers 35 and 37. The difference is between them is 2.

sorry i still don't get how to answer it :/

Reply 4

pureandmodest

Original post by Petulia

Cl-35 is more abundant, so this is the value you use.
Then, you just calculate the Mr of the whole molecule: C12H4O2Cl4
C: 12 x 12
H: 4 x 1
O: 16 x 2 altogether = 182
182 + Cl: 35 x 4 = 322

use for what?
i don't get it, surely you'd do the same thing if you used the other chlorine
do you know how to work out the number of molecular ion peaks?thankyou :smile:

Reply 5

EierVonSatan

Original post by pureandmodest

sorry i still don't get how to answer it :/

Do you know how to calculate a molecular mass?

Do you know the difference between relative atomic mass and mass number?

Reply 6

Pigster

The molecular ion peak is the peak on the MS for the entire molecule (as an ion). It is the right-handest peak, the heaviest one. In this case, there are several molecules that could be considered the heaviest, since there are 4xCl and each Cl can be either the 35 or the 37 isotope.

The base of the molecule (i.e. the bit we know the mass of) is C12H4O2 i.e. 12x12 + 4x1 + 16x2 = 180

If all four Cls were 35, the entire mass would be 180 + 4x35 = 320
If three were 35 and one were 77... 180 + 3x35 + 37 = 322
2x35 + 2x37... 324
etc.
But, given the proportions in the Q, you know 25% are 37, hence 322 is the best answer.