Question

Bromine occurs in nature mainly in the form of two isotopes ${}_{35}{}^{79}\mathrm{Br}$ and ${}_{35}{}^{81}\mathrm{Br}$. If the abundance of ${}_{35}{}^{79}\mathrm{Br}$ is isotope is 49.7% and that of ${}_{35}{}^{81}\mathrm{Br}$​ isotope is 50.3% calculate the average atomic mass of bromine.

Answer 1

$$\begin{gathered} \mathrm{Isotope}\mathrm{of}\mathrm{bromine}\mathrm{with}\mathrm{atomic}\mathrm{mass}79\mathrm{is}49.7\%. \\ \mathrm{So},\mathrm{it}\text{'}\mathrm{s}\mathrm{contribution}\mathrm{to}\mathrm{the}\mathrm{atomic}\mathrm{mass}\mathrm{of}\mathrm{bromine}=79\times \frac{49.7}{100} \\ =39.263\mathrm{u} \\ \mathrm{Isotope}\mathrm{of}\mathrm{bromine}\mathrm{with}\mathrm{atomic}\mathrm{mass}81\mathrm{is}50.3\%. \\ \mathrm{So},\mathrm{it}\text{'}\mathrm{s}\mathrm{contribution}\mathrm{to}\mathrm{the}\mathrm{atomic}\mathrm{mass}\mathrm{of}\mathrm{bromine}=81\times \frac{50.3}{100} \\ =40.743\mathrm{u} \\ \mathrm{Average}\mathrm{atomic}\mathrm{mass}\mathrm{of}\mathrm{Bromine}=39.263+40.743 \\ =80.006\mathrm{u} \end{gathered}$$