Question

HELP: Nitrogen has two isotopes, N-14 and N-15, with atomic masses of 14.00031 amu and 15.001 amu, respectively. What is the percent abundance of N-15?

Answer 1

The atomic mass of an element is a weighted average of its isotopes in which the sum of the abundance of each isotope is equal to 1 or 100%.

Let fraction of N-15 be x
then fraction of N-14 = 1-x 

Formula for Calculating Isotope:

$15.001x + 14.00031(1-x) = 14.0067$

on calculation:

x = 0.006403
so, % = 0.006403 x 100  =  0.6403 %