Europium has two stable isotopes, 151 Eu (150.9198 u) and 153 Eu (152.9212 u). The atomic mass given for Eu is 151.9640 u. What is the percent abundance of 153 Eu ??

Answer 1

y = fraction Eu 151
then 1-y = fraction Eu 153
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y(150.9198) + (1-y)(152.9212) = 151.9640
Solve for y and 1-y.