# Problem: 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 amuB. 79.18 amuC. 64.92 amuD. 65.22 amuE. 69.17 amu

###### FREE Expert Solution

We’re being asked to determine the atomic mass of 65X.

We can use the following equation:

where:

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

88% (446 ratings) ###### Problem Details

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