## Chemistry: Molecular Approach (4th Edition)

$\underline{20\,\text{percent}}$ and $80\,\text{percent}$
Calculate the fraction of boron-10 as follows: $\text{Fraction of isotope boron-}10=\frac{\text{Relative abundance of boron-}10}{\text{100}}$ Calculate the fraction of boron-11 as follows: $\text{Fraction of isotope boron-}11=\frac{\text{Relative abundance of boron-}11}{\text{100}}$ Atomic mass is the average of the mass of isotopes. So, the atomic mass of boron is $10.81\text{ amu}$. Calculate the relative abundance as follows: \begin{align} & \text{Atomic mass}=\left( \text{Fraction of boron-}10\times \text{Mass of boron-}10 \right)+ \\ & \left( \text{Fraction of boron-}11\times \text{Mass of boron-}11 \right) \\ & 10.81=\left( \frac{\text{Relative abundance of boron-}10}{\text{100}}\times 10.01294\text{ amu} \right) \\ & +\left( \frac{\text{Relative abundance of boron-}11}{\text{100}}\times 11.00931\text{ amu} \right) \end{align} Substitute relative abundance of boron-11 as $100-\text{relative abundance of boron-}10$ in the above expression as follows: \begin{align} & 10.81=\left( \frac{\text{Relative abundance of boron-}10}{\text{100}}\times 10.01294\text{ amu} \right)+ \\ & \left( \frac{\text{100}-\text{Relative abundance of boron-}10}{\text{100}}\times 11.00931\text{ amu} \right) \end{align} $\text{Relative abundance of boron}-10=20\,\text{percent}$ Calculate the relative abundance of boron-11 as follows: \begin{align} & \text{Relative abundance of boron-}11=100\,\text{percent}-\underline{20\,\text{percent}} \\ & =80\,\underline{\text{percent}} \end{align} The relative abundance of boron-10 is $\underline{20\,\text{percent}}$ and relative abundance of boron-11 is $80\,\text{percent}$.