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Need help with atomic relative mass question?

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The relative atomic mass of iridium is 192.2

irdium occurs naturally as a mixture of irdium 191 and irdium 193. work out the percentage of each isotope in naturally occuring irrdium.

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  1. You know that the two isotopes must have an average mass of 192.2

    So set up equation which can solve for this like

    Let decimal fraction of 191 isotope be x

    So decimal fraction of 193 isotope is 1- x

    And 191x + 193 (1-x) = 192.2

           191x + 193 - 193x = 192.2

    Gives   2x = 0,8

               x = 0.4

    Ask is it reasonable? Yes it is

    40% 191  60% 193


  2. There is a proportion, x, of all Iridium that is 191.  The remainder (1-x) is 193.  The total, x*191+(1-x)*193 = 192.2.  You need to solve this for x, i.e. 191x-193x+193=192.2

    -2x=-0.8

    x=0.4

    so 40% of all iridium is 191 (60% is 193).

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