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?

Answer 1

Take the weighted average of the individual isotopes.

Explanation:

`""^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 the Periodic Table, copper metal (a mixture of isotopes but `""^63Cu` and `""^65Cu` predominate) has an approximate atomic mass of `63.55` `g*mol^-1`, so we know we are right.