## Chemistry: Molecular Approach (4th Edition)

$\underline{\text{78}\text{.92 amu}}$ and $\underline{\text{50}\text{.69}\,\text{percent}}$
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 $\underline{\text{50}\text{.69}\,\text{percent}}$.