Final answer:
The atomic mass of copper is not exactly 64 due to the presence of two isotopes, Cu-63 and Cu-65, with different masses and abundances. The average atomic mass is calculated by taking the weighted average of the masses of the isotopes, considering their natural abundance.
Step-by-step explanation:
The atomic mass of copper is not exactly equal to 64 because copper occurs in nature in the form of two isotopes, Cu-63 and Cu-65, with different masses. The atomic mass of an element is the weighted average of the masses of its isotopes, taking into account their natural abundance.
In the case of copper, Cu-63 has a mass of 62.9296 amu and an abundance of 69%, while Cu-65 has a mass of 64.9278 amu and an abundance of 31%. By calculating the weighted average, the average atomic mass of copper is determined to be 63.546 amu.
The percent composition of copper in terms of these two isotopes can be calculated by dividing the mass of each isotope by the average atomic mass and multiplying by 100. For Cu-63, the percent composition is (62.9296 amu / 63.546 amu) * 100 = 98.29%. For Cu-65, the percent composition is (64.9278 amu / 63.546 amu) * 100 = 101.71%. These values add up to 100%, representing the composition of copper in a naturally occurring sample.