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QUESTION

# The element copper has naturally occurring isotopes with mass numbers of 63 and 65. The relative abundance and atomic masses are 69.2% for a mass of 62.93 amu and 30.8% for a mass of 64.93 amu. What is the average atomic mass of copper?

Take the weighted average of the individual .

""^63Cu has 69.2% abundance.

""^65Cu has 30.8% abundance.

So, the weighted average is 62.93xx69.2% + 64.93xx30.8% = 63.55 "amu".

If we look at , copper metal (a mixture of isotopes but ""^63Cu and ""^65Cu predominate) has an approximate of 63.55 g*mol^-1, so we know we are right.