We’re being asked to determine the atomic mass of 65X.
We can use the following equation:
atomic mass = average atomic mass of the element
f.a. = fractional abundance of the isotope.
To get f.a., we simply need to divide the given percent abundance by 100.
We’re given the following values:
Average Atomic Mass = 63.55 amu
63X: Mass = 62.9396 amu
f.a. = 69.17/100 = 0.6917
We don’t know the percent abundance of either isotope but we can use the fact that the f.a. of all isotopes of an element add up to 1.
65X: Mass = ???
f.a. = 1 - 0.6917 = 0.3083
The naturally occurring isotopes of element X consists of the isotopes 63X, and 65X. The atomic mass of 63X is 62.9396 amu with a percent abundance of 69.17%. With an average atomic mass of X of 63.55 amu, the atomic mass of 65X is:
A. 64.44 amu
B. 79.18 amu
C. 64.92 amu
D. 65.22 amu
E. 69.17 amu
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