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Homework Help: Calculate fraction of relative molecular mass

  1. May 20, 2012 #1
    Can anyone help with the following question - see attachment.
    Answer C is correct.
    I have been told that it can be solved thus:

    As our compound contains carbon and hydrogen we need to calculate these masses and deduct them from 128 to give us the mass of the chlorine and bromine combination.

    2H = 2 and C = 12.

    128 - 14 = 114. This is the remaining mass of the chlorine and bromine.

    The only combination of isotopes that give us this value are Cl 35 and Br 79.

    We know that the chance of Cl being 35 is 3:4 or ¾ and the chance of Br being 79 is ½.
    If we now multiply these fractions we find the required fraction of the compound.

    ¾ x ½ = 3/8. Answer C is correct.

    However what I do not understand is how this gives the correct fraction. The mass of the compound is (12) + (2) + [(3/4 x 35) + (1/4 x 37)] + [(1/2 x 79) + (1/2 + 81)] = 129.5

    Therefore the fraction of the mass of 128 to the total mass of 129.5 = 99% approx. Even if I just select the elements with a combined mass of 128 i.e. 12 + 2 + 26.25 + 39.5 = 79.75, this gives me 79.75/129.5 = 61.5% approx. which is not 3/8.

    What I also do not understand is why the fractions are multiplied and why the mass of the hydrogen and carbon (which must be added to give a mass of 128) do not affect this fraction.

    Can anyone help? All replies greatly appreciated.



    Attached Files:

  2. jcsd
  3. May 20, 2012 #2


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    Staff: Mentor

    Yes, this molar mass, averaged over all possible combinations.

  4. May 20, 2012 #3
    Hi PFAdmin,

    Thanks very much. I had become so confused I couldn't see the wood for the trees. Your explanation is very clear. Thanks again.

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