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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.

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Solution

## $\mathrm{Isotope}\mathrm{of}\mathrm{bromine}\mathrm{with}\mathrm{atomic}\mathrm{mass}79\mathrm{is}49.7%.\phantom{\rule{0ex}{0ex}}\mathrm{So},\mathrm{it}\text{'}\mathrm{s}\mathrm{contribution}\mathrm{to}\mathrm{the}\mathrm{atomic}\mathrm{mass}\mathrm{of}\mathrm{bromine}=79×\frac{49.7}{100}\phantom{\rule{0ex}{0ex}}=39.263\mathrm{u}\phantom{\rule{0ex}{0ex}}\mathrm{Isotope}\mathrm{of}\mathrm{bromine}\mathrm{with}\mathrm{atomic}\mathrm{mass}81\mathrm{is}50.3%.\phantom{\rule{0ex}{0ex}}\mathrm{So},\mathrm{it}\text{'}\mathrm{s}\mathrm{contribution}\mathrm{to}\mathrm{the}\mathrm{atomic}\mathrm{mass}\mathrm{of}\mathrm{bromine}=81×\frac{50.3}{100}\phantom{\rule{0ex}{0ex}}=40.743\mathrm{u}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\mathrm{Average}\mathrm{atomic}\mathrm{mass}\mathrm{of}\mathrm{Bromine}=39.263+40.743\phantom{\rule{0ex}{0ex}}=80.006\mathrm{u}$

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