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QUESTION

# 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.