Answer: Correct option is B.
Explanation: In this question, we are given two isotopes of Boron.
Mass of isotope [tex]B^{10}=10amu[/tex]
Mass of isotope [tex]B^{11}=11amu[/tex]
Fractional abundance can be calculated as:
[tex]\text{Fractional abundance}=\frac{\% abundance}{100}[/tex]
Fractional abundance of isotope [tex]B^{10}=\frac{19.80}{100}=0.198[/tex]
Fractional abundance of isotope [tex]B^{11}=\frac{80.20}{100}=0.802[/tex]
To calculate average atomic mass of an element, we use:
[tex]\text{Average Atomic Mass}=\sum_{i=1}^{n}(\text{Fractional abundance})_i\times (\text{Mass number})_i[/tex]
Now, putting the values of abundances and mass number of 2 isotopes in above equation, we get:
[tex]\text{Average Atomic mass of B}=(0.198\times 10amu)+(0.802\times 11amu)[/tex]
Average Atomic Mass of B = 10.802 amu
Rounding it off to 3 significant figures, we get
Average atomic mass of B = 10.8 amu.
