Question

1. The element copper has naturally occurring isotopes with mass numbers of 63 and 65. The relative abundance and atomic masses are 69.2% for a mass of 62.93amu and 30.8% for a mass of 64.93amu. Calculate the average atomic mass of copper.

Answer 1

Answer: The average atomic mass of copper is 63.546 amu

Explanation:

Average atomic mass of an element is defined as the sum of masses of each isotope each multiplied by their natural fractional abundance.

Formula used to calculate average atomic mass follows:

$\text{Average atomic mass }=\sum_{i=1}^n\text{(Atomic mass of an isotopes)}_i\times \text{(Fractional abundance})_i$  .....(1)

• For isotope 1 (Cu-63):

Mass of isotope 1 = 62.93 amu

Percentage abundance of isotope 1 = 69.2 %

Fractional abundance of isotope 1 = 0.692

• For isotope 2 (Cu-65):

Mass of isotope 2 = 64.93 amu

Percentage abundance of isotope 2 = 30.8 %

Fractional abundance of isotope 2 = 0.308

Putting values in equation 1, we get:

$\text{Average atomic mass of Copper}=[(62.93\times 0.692)+(64.93\times 0.308)]\\\\\text{Average atomic mass of Copper}=63.546amu$

Hence, the average atomic mass of copper is 63.546 amu