Chemistry: Molecular Approach (4th Edition)

$\underline{\text{85}\text{.46 amu}}$
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 $\underline{\text{85}\text{.46 amu}}$.