Answer
\[\underline{\text{78}\text{.92 amu}}\] and \[\underline{\text{50}\text{.69}\,\text{percent}}\]
Work Step by Step
Calculate the Br-81 fraction as follows:
\[\begin{align}
& \text{Fraction of isotope Br-}8\text{1}=\frac{49.31}{100} \\
& =0.4931
\end{align}\]
Calculate the Br-79 natural abundance as follows:
\[\begin{align}
& \text{Natural abundance}=\text{100}\,\text{percent}-\text{Natural abundance of Br-81} \\
& =\text{100}\,\text{percent}-\text{49}\text{.31}\,\text{percent} \\
& =\text{50}\text{.69}\,\text{percent}
\end{align}\]
Calculate the Br-79 fraction as follows:
\[\begin{align}
& \text{Fraction of isotope Br-79}=\frac{50.69}{100} \\
& =0.5069
\end{align}\]
Calculate the atomic mass of element as follows:
\[\begin{align}
& \text{Atomic mass}=\left( \text{Fraction of Br-81}\times \text{Mass of Br-81} \right)+ \\
& \left( \text{Fraction of Br-79}\times \text{Mass of Br-79} \right) \\
& \text{79}\text{.904 amu}=\left( \text{0}\text{.4931}\times \text{80}\text{.9163 amu} \right)+\left( \text{0}\text{.5069}\times \text{Mass of Br-79} \right) \\
& \text{79}\text{.904 amu}=\text{39}\text{.8998 amu}+\left( \text{0}\text{.5069}\times \text{Mass of Br-79} \right) \\
& \text{Mass of Br-79}=78.9193\text{ amu}\approx \,78.\text{92 amu}
\end{align}\]
The mass of Br-79 is \[\underline{\text{78}\text{.92 amu}}\] and the natural abundance is \[\underline{\text{50}\text{.69}\,\text{percent}}\].
