# How would you calculate the average atomic mass of copper if 69.2% of copper has a mass of 62.93 amu and 30.8% has a mass of 64.93 amu?

##### 1 Answer

#### Explanation:

The **average atomic mass** of an element is calculated by taking the **weighted average** of the atomic masses of its naturally-occurring isotopes.

Simply put, each isotope will contribute to the average atomic mass of the element **proportionally** to its *percent abundance*.

#color(blue)("avg. atomic mass" = sum_i ("isotope"_i xx "abundance"_i)#

As far as the actual calculations go, you will use *decimal abundances*, which are simply percent abundances divided by

So, you know that the atomic masses of these two copper isotopes are *decimal abundances* will be

The average atomic mass of copper will thus be

#"avg. atomic mass" = overbrace("62.93 u" xx 0.692)^(color(red)(1^"st" "isotope")) + overbrace("64.93 u" xx 0.308)^(color(red)(2^"nd" "isotope"))#

#"avg. atomic mass " = color(green)(" 63.546 u")#

I'll leave the answer is rounded to four sig figs, despite the fact that the values you have for the percent abundances justify only three sig figs.