Element x is composed of isotopes x-56 mass 56.0 u and x-59 mass 59.0 u. What is the percentage abundance of each of these isotopes if the atomic mass is 57.3 u?

Answer 1

Let f = fraction of x-56,
then 1-f = fraction of x=59
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56f + 59(1-f) = 57.3
solve for f and 1-f and convert each to percent.
Note: (we could have said f = % of 56 and 100-f - % of 59 but then we would have to multiply 57.3 by 100 and solve for f and 100-f; the answer would have been directly in percent.)