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    Not sure how to work this out even though it's only worth one mark.
    Bromine has two isotopes BR-79 and Br-81 the relative atomic mass of br is 79.9 calculate the percentage of br-79 atoms in a sample of bromine.
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    Well on the periodic table, Bromine has a mass number of 79.9

    So...

    x(81) + (1-x)(79) = 79.9
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    (Original post by High Stakes)
    Well on the periodic table, Bromine has a mass number of 79.9

    So...

    x(81) + (1-x)(79) = 79.9


    How did u work that out?
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    (Original post by medicaesthetic)
    How did u work that out?
    It's how averages work - The numbers you see on the periodic table are averages.

    When you take: 2 + 2 + 3 + 3 and divide it by 4, you're doing the same as (0.5 x 2) + (0.5 x 3).

    So, when it comes to this we know there can only be two isotopes. We call one of their abundances x and we call the other 1-x because that's the remaining abundance.
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    (Original post by High Stakes)
    It's how averages work - The numbers you see on the periodic table are averages.

    When you take: 2 + 2 + 3 + 3 and divide it by 4, you're doing the same as (0.5 x 2) + (0.5 x 3).

    So, when it comes to this we know there can only be two isotopes. We call one of their abundances x and we call the other 1-x because that's the remaining abundance.
    I've never seen it like that could you not just put the numbers in ;s
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    (Original post by medicaesthetic)
    I've never seen it like that could you not just put the numbers in ;s
    Are you AS Chemistry? This should have been covered in like the first few lessons.
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    (Original post by High Stakes)
    Are you AS Chemistry? This should have been covered in like the first few lessons.
    Ye I've been getting 90/80%'s all year round and this question I don't have a clue lol, I don't think we've been taught it.
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    You did not answer the question though you just worked out the average where the question is asking you to work out the abundance of Br-79.
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    (Original post by High Stakes)
    Well on the periodic table, Bromine has a mass number of 79.9

    So...

    x(81) + (1-x)(79) = 79.9
    you did not answer their question.The question is asking you to work out the percentage of Br-79 in the bromine sample,where you just worked out the average.
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    The answer is 55%. If you were given the % of each isotope and asked to calculate the relative atomic mass, you would do the following

    (79x55+81x45)/100 = 79.9

    Now, in the question you are asked what is the % of Br-79, so replace with "x" (and replace the Br-81 % with "100-x" as both added together must equal 100%).
    (79x + 81(100-x))/100 =79.9
    79x + 81(100-x)=7990
    79x +8100 -81x =7990
    -2x=-110 x=55

    Therefore Br-79 55%
 
 
 

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