Question

Boron has two isotopes B - 10 and B - 11. If the average atomic mass of boron is 10.80 u. What is the percentage of a heavier isotope?

• A. 80
• B. 20
• C. 30
• D. 70

Answer 1

Answer: (A)

80

Let B-10 = x%

and B-11 = (100-x)%

average atomic mass = 10x + (100 - x)/100 = 10.80

10x + 11(100 - x) = 1080

10x + 1100 - 11x = 1080

x = 20

B-10 = 20% and B-11 = (100-20) = 80%

Thus, the percentage of a heavier isotope is 80%.

Hence, the correct option is $\text{A}$