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    Q11) I thought a compound can only have 1 molecular ion peak?
    Q12) I know the formula for atom economy but none of them give the correct value? :s
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    Since you have two isotopes of Bromine present, you will have more than one molecular ion peak.

    Not sure what you mean by atom economy.
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    (Original post by sabre2th1)


    Q11) I thought a compound can only have 1 molecular ion peak?
    Q12) I know the formula for atom economy but none of them give the correct value? :s
    11) As there are two Br isotopes to consider and two Br in each molecule, you could have:

    a) one with 79Br and 79Br
    b) one with 79Br and 81Br
    c) one with 81Br and 81Br

    So you would have three peaks for the molecular ion.

    There are often other peaks present around the molecular ion for an organic molecule (eg for when some carbon 13 is present), but the abundance of other isotopes is so small that these peaks are only tiny. For bromine the two isotopes have a bigger abundance relative to each other, hence you notice the peaks due to differing isotopes.

    12) Remember it's (sum of Mr of useful products/sum of Mr of all products)x100
    and make sure you've got the correct product as your useful product (a very easy mistake to make on these). It does work.
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    (Original post by clownfish)
    11) As there are two Br isotopes to consider and two Br in each molecule, you could have:

    a) one with 79Br and 79Br
    b) one with 79Br and 81Br
    c) one with 81Br and 81Br

    So you would have three peaks for the molecular ion.

    There are often other peaks present around the molecular ion for an organic molecule (eg for when some carbon 13 is present), but the abundance of other isotopes is so small that these peaks are only tiny. For bromine the two isotopes have a bigger abundance relative to each other, hence you notice the peaks due to differing isotopes.

    12) Remember it's (sum of Mr of useful products/sum of Mr of all products)x100
    and make sure you've got the correct product as your useful product (a very easy mistake to make on these). It does work.
    Oh I see.. How comes we don't consider the 1H and 12C isotopes?

    Regarding Q12: I understand where I went wrong now! Whenever there is one product only, the atom economy is always 100% right?

    Thanks!
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    (Original post by Ari Ben Canaan)
    Since you have two isotopes of Bromine present, you will have more than one molecular ion peak.

    Not sure what you mean by atom economy.
    How do you know its the isotopes of bromine that affect the number of molecular ion peaks? (rather than H or C isotopes)?
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    (Original post by sabre2th1)
    How do you know its the isotopes of bromine that affect the number of molecular ion peaks? (rather than H or C isotopes)?
    The question states that we should only consider one isotope of Carbon, one isotope of Hydrogen and TWO isotopes of Bromine.

    That means the Mr of the compound can be :

    12*2+4+79*2

    or

    12*2+4+81*2

    or

    12*2+4+79+81

    These are the there molecular ion peaks.
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    (Original post by sabre2th1)
    Oh I see.. How comes we don't consider the 1H and 12C isotopes?

    Regarding Q12: I understand where I went wrong now! Whenever there is one product only, the atom economy is always 100% right?

    Thanks!
    Ari has answered the first part for you. In general remember the abundance of the other isotopes of carbon and hydrogen is very small in comparision to C-12 and H-1.

    re: the atom economy, absolutely if only one product the AE is 100%.
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    (Original post by Ari Ben Canaan)
    The question states that we should only consider one isotope of Carbon, one isotope of Hydrogen and TWO isotopes of Bromine.

    That means the Mr of the compound can be :

    12*2+4+79*2

    or

    12*2+4+81*2

    or

    12*2+4+79+81

    These are the there molecular ion peaks.

    (Original post by clownfish)
    Ari has answered the first part for you. In general remember the abundance of the other isotopes of carbon and hydrogen is very small in comparision to C-12 and H-1.

    re: the atom economy, absolutely if only one product the AE is 100%.
    Oh! I understand now. Thanks a lot

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Updated: May 22, 2012
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