Boron occurs in the nature in the form of two isotopes $_{5}^{11} B$ and $_{5}^{10} B$ in ratio of $81 %$ and $19 %$ respectively. Calculate its average atomic mass.

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Solution

The average atomic weight can be calculated by using the formula:

Average atomic weight of the element = (exact weight of isotope 1) $\times$ (abundance of isotope 1) + (exact weight of isotope 2) $\times$ (abundance of isotope 2)
Let average atomic weight of the element $= x$ .
The abundance of $B^{10}$ be assigned '19%' and the $B^{11}$ be 81%.

So, $x = (0.19) (10) + (0.81) (11)$

$x = 1.9 + 8.91$

$x = 10.81$

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Boron occurs in the nature in the form of two isotopes $_{5}^{11} B$ and $_{5}^{10} B$ in ratio of $81 %$ and $19 %$ respectively. Calculate its average atomic mass.