Question

The element bromine exists in nature as two isotopes: 79Br has a mass of 78.9183 u, and 81Br has a mass of 80.9163 u. The average atomic mass of bromine is 79.90 u. Calculate the relative abundance (as percentages) of the two bromine isotopes.

Answer 1

Abundance of isotope 79Br=51% and abundance of 81Br=49%

Explanation:

We are given that

Mass of 79Br=78.9183 u

Mass of 81Br=80.9163 u

Average atomic mass of bromine=79.90 u

We have to find the relative abundance of the two bromine isotopes.

Let x be the abundance of 79Br

Then , abundance of 81Br isotopes=1-x

Average atomic mass=Mass of isotope(abundance of isotope)+Mass of other isotope(abundance of other isotope)

Using the formula

`79.90=78.9183x+(1-x)80.9163`

`79.90=78.9183x+80.9163-80.9163x`

`79.90-80.9163=-1.998x`

`-1.0163=-1.998x`

`x=(1.0163)/(-1.998)=0.51`

Percentage of isotope with abundance 0.51=
`0.51* 100=51`%

Abundance of other isotope=1-0.51=0.49

Percentage of other isotope with abundance 0.49=
`0.49* 100=49`%