Answer
% abundance of $^{63}Cu$ in naturally occurring copper is $69.16\%$
Work Step by Step
Atomic mass of Copper $=63.546amu$
Mass of $^{63}Cu = 62.9298amu$
Mass of $^{65}Cu = 64.9278amu$
Let the fraction of $^{63}Cu = x$
The fraction of $^{65}Cu = (1-x)$
The atomic mass of an element is the weighted average of the masses of the constituent isotopes. Therefore,
Atomic mass of Copper $= x(62.9298amu)+(1-x)(64.9278amu)$
$63.546=62.9298x+64.9278-64.9278x$
$-1.3818=-1.998x$
$x=0.6916$
The fraction of $^{63}Cu = 0.6916$
% abundance of $^{63}Cu = 69.16\%$
