Question

Boron has two stable isotopes, $^{10}B (19 \%)$ and $^{11}B (81\%)$. Calculate average atomic weight of boron in the periodic table.

• A. 10.8
• B. 10.2
• C. 11.2
• D. 10

Answer 1

Answer: (A)

10.8

Answer 2

Average of atomic weight
$\% \, of \, B \times \, atomic \, mass\, of \, ^{10}B + \% \, of \, ^{11}B$
$= \frac {\times \, atomic \, mass\, of \, ^{11}B}{\%\, of \, ^{10}B + \% of ^{11}B }$
$\frac {19 \times 10 + 81 \times 11}{19 + 81}$
$\frac {190 \times 891}{100} = 10.81$