Answer
\[\underline{\text{85}\text{.46 amu}}\]
Work Step by Step
Calculate the abundance of Rb-85 as follows:
\[\begin{align}
& \text{Abundance of Rb-}85=\frac{100\,\text{percent}}{100\,\text{percent}+35\,\text{percent}}\times 100\,\text{percent} \\
& =74.07\,\text{percent}
\end{align}\]
Calculate the abundance of Rb-87 as follows:
\[\begin{align}
& \text{Abundance of Rb-}87=\frac{35\,\text{percent}}{100\,\text{percent}+35\,\text{percent}}\times 100\,\text{percent} \\
& =25.93\,\text{percent}
\end{align}\]
Calculate the Rb-85 fraction as follows:
\[\begin{align}
& \text{Fraction isotope Rb-}85=\frac{74.07}{100} \\
& =0.7407
\end{align}\]
Calculate the Rb-87 fraction as follows:
\[\begin{align}
& \text{Fraction isotope Rb-}87=\frac{25.93}{100} \\
& =0.2593
\end{align}\]
Calculate the atomic mass of the element as follows:
\[\begin{align}
& \text{Atomic mass}=\left( \text{fraction of Rb-}85\times \text{mass of Rb-}85 \right)+ \\
& \left( \text{fraction of Rb-}87\times \text{mass of Rb-}87 \right) \\
& =\left( 0.7407\times 85\text{ amu} \right)+\left( 0.2593\times 87\text{ amu} \right) \\
& =62.93\text{ amu}+22.53\text{ amu} \\
& =85.46\text{ amu}
\end{align}\]
The atomic mass of rubidium is \[\underline{\text{85}\text{.46 amu}}\].