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# 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?

##"63.546 u"##

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

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 ##100##.

So, you know that the atomic masses of these two copper isotopes are ##"62.93 u"## and ##"64.93 u"##, respectively. Their decimal abundances will be ##0.692## and ##0.308##, respectively.

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 , despite the fact that the values you have for the percent abundances justify only three sig figs.