# A sample of copper (Cu) contains 69.17% Cu-63. The remaining copper atoms are Cu-65. What is the average atomic mass of copper?

Feb 4, 2017

$\text{Average atomic mass = 63.62 amu}$

#### Explanation:

$\text{Average atomic mass}$ is the weighted average of the individual isotopes.

And thus the weighted average $=$

$\left(0.6917 \times 63 + 0.3083 \times 65\right) \cdot \text{amu}$ $=$ $\text{?? amu}$.

Note that we have not accounted for the less massive copper isotopes that would decrease this mass to the accepted value of $63.55$ $\text{amu}$.