Question

Average atomic masses listed by IUPAC are based on a study of experimental results. Bromine has two isotopes 73 br and81 br, whose masses (78.9183 and 80.9163 amu) and abundances (50.69% and 49.31%) were determined in earlier experiments. Calculate the average atomic mass of bromine based on these experiments.

Answer 1

Answer: The average atomic mass of element bromine is 80.4104 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 _{35}^{79}\textrm{Br}[/tex] isotope:

Mass of $_{35}^{79}\textrm{Br}$ isotope = 78.9183 amu

Percentage abundance of $_{35}^{79}\textrm{Br}$ isotope = 50.69 %

Fractional abundance of $_{35}^{79}\textrm{Br}$ isotope = 0.5069

• For $_{35}^{81}\textrm{Br}$ isotope:

Mass of $_{35}^{81}\textrm{Br}$ isotope = 80.9163 amu

Percentage abundance of $_{35}^{81}\textrm{Br}$ isotope = 49.31 %

Fractional abundance of $_{35}^{81}\textrm{Br}$ isotope = 0.4931

Putting values in equation 1, we get:

$\text{Average atomic mass of Bromine}=[(78.9183\times 0.5069)+(80.9163\times 0.4931)]$

$\text{Average atomic mass of Bromine}=80.4104amu$

Hence, the average atomic mass of element bromine is 80.4104 amu.