Answer: The correct answer is Option A.
Explanation:
Average atomic mass of an element is defined as the sum of masses of each isotope each multiplied by their natural fractional abundance.
Formula used to calculate average atomic mass follows:
[tex]\text{Average atomic mass }=\sum_{i=1}^n\text{(Atomic mass of an isotopes)}_i\times \text{(Fractional abundance})_i[/tex]
We are given:
Mass of isotope 1 = 267.8 u
Percentage abundance of isotope 1 = 90.3 %
Fractional abundance of isotope 1 = 0.903
Mass of isotope 2 = 270.9 u
Percentage abundance of isotope 2 = 9.7%
Fractional abundance of isotope 2 = 0.097
Putting values in above equation, we get:
[tex]\text{Average atomic mass of Z}=[(267.8\times 0.903)+(270.9\times 0.097)][/tex]
[tex]\text{Average atomic mass of Z}=268.1u[/tex]
Thus, the correct answer is Option A.
