Question

How do I calculate the average atomic mass?

Answer 1

To calculate the average atomic mass, multiply the fraction by the mass number for each isotope, then add them together. Whenever we do mass calculations involving elements or compounds, we always use average atomic masses.

Complete step by step solution:
The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its natural abundance. Average atomic mass can be found on the periodic table.
We have the formula to calculate the average atomic mass:
$Avg.\,Atomic\,mass = \sum (isotope\,Mass \times Natural\,Abundance)$
As for example: Consider the chlorine isotopes, chlorine -35 has a mass of $34.969amu$ , while chlorine-37 has a mass of $36.966{\text{ }}amu$ , if their natural abundance is $75.77\% {\text{ }}and{\text{ }}24.23\%$ respectively, calculate their average atomic mass.
So, first we will calculate the atomic mass of chlorine- 35:
$Chlorine - 35 \\\ = 34.969 \times 0.7577 \\\ = 26.50 \\\$
And,
$Chlorine - 37 \\\ = 36.966 \times 0.2423 \\\ = 8.957 \\\$
So, the average atomic mass is-
$\therefore Avg.\,atomic\,mass \\\ = 26.50 + 8.957 \\\ = 35.457amu \\\$
Therefore, our average atomic mass is $35.457amu$ .

Note:
The atomic mass is the weighted average of the atomic masses of each isotope. In a weighted average, we multiply each value by a number representing its relative importance. It is one of the essential topic that is covered in chemistry.