What is the average atomic mass of bromine? One isotope of bromine has an atomic mass of 78.92amu and a relative abundance of 50.69%. The other major isotope of bromine has an atomic mass of 80.92amu and a relative abundance of 49.31%.
1 Answer
The average atomic mass of Br is 79.91 u.
There are two ways to solve this problem.
Method 1
Let's assume we have 10 000 atoms of Br. Then 5069 atoms will have mass 78.92 u, and 4931 atoms will have mass 80.92 u.
Mass of the 5069 atoms = 5069 × 78.92 u = 400 000 u
Mass of the 4931 atoms = 4931 × 80.92 u = 399 000 u
Mass of 10 000 atoms = 799 100 u
Average mass of an atom =
Method 2
The average mass is the mass of each isotope multiplied by its percentage. Thus,
Average mass = 50.69 % × 78.92 u + 49.31 % × 80.92 u = 40.00 u + 39.90 u = 79.91 u
The second method is easier, but it doesn't explain intuitively why the answer is the average atomic mass.
Here is a video which summarizes the steps needed to calculate average atomic mass.
