Copper consists of two naturally occuring isotopes. their masses are 62.930 amu and 64.928 amu. calculate their abundances.

The atomic mass of copper on the periodic table is a mixture of all the stable isotopes of the element.  In short, it is a weighted average.

The atomic mass for Cu is 63.546 amu.  This number is calculated by multiplying the masses of the isotopes by their percent occurrence or abundance in nature. For copper, it would be as follows:

atomic mass = ( Isotope 1 x % abundance) + ( Isotope 2 + % abundance)

We are given the mass of both the isotopes.  We only need to look up the weighted average of their weights or the atomic mass which is already mentioned above. Substituting what is given to the equation:

63.546 amu = (62.930 amu ( x )) + (64.928 amu ( 1 - x ))

where x is the percent abundance of the first isotope. Calculating for x, we will have

x =0.69 = 69%
1-x = 0.31 = 31%