Question

In a naturally occurring sample, 69.2% of copper atoms have 34 neutrons and 30.8% have 36 neutrons. What is the average mass of the atoms in the drawing? (Copper-63 has a mass of 62.92960 amu, and copper-65 has a mass of 64.92779 amu.) Express your answer in atomic mass units to five decimal places.

Answer 1

So, remember that:

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.

We could use the following formula:

`avg.\text{ atomic mass =}\sum ^{}_i(mass(_{}isotope)_i\cdot abundance_i)`

We're given that the decimal abundances for each isotope are 0.962 (copper 63) and 0.308 (copper 65).

Given that the mass of each isotope is 62.92960 amu and 64.92779 amu respectively, we can replace in the formula:

`\begin{gathered} avg.atomic.mass=(62.92960amu)(0.692)+(64.92779amu)(0.308) \\ \text{avg. atomic mass }=63.54504\text{amu} \end{gathered}`

Therefore, the average mass is 63.54504 amu.