Answer:
[tex]\boxed{\text{10.81 u}}[/tex]
Explanation:
The atomic mass of B is the weighted average of the atomic masses of its isotopes.
We multiply the atomic mass of each isotope by a number representing its relative importance (i.e., its percent of the total).
Set up a table for easy calculation:
[tex]\begin{array}{ccccr}\textbf{Atom} & \textbf{Mass/u} &\textbf{Percent} & \textbf{Calculation}& \textbf{Result}\\^{10}\text{B}& 10.01 & 19.91 & 10.01 \times 0.1991 & 1.99\\^{11}\text{B}& 11.01 & 80.09 & 11.01 \times 0.8009 & 8.82 \\& & & \text{TOTAL} = &\textbf{10.81}\\\end{array}[/tex]
The average atomic mass of B is [tex]\boxed{\textbf{10.81 u}}[/tex].
