Question Detail
Question
The average atomic mass of lithium is 6.94 amu. It has two isotopes: ⁶Li (6.015 amu) and ⁷Li (7.016 amu). What is the percent abundance of ⁷Li?
Correct Answer
Option C is the correct answer.
Detailed Explanation
Let x = fraction of ⁷Li, then (1-x) = fraction of ⁶Li. 6.94 = (1-x)(6.015) + x(7.016). 6.94 = 6.015 - 6.015x + 7.016x = 6.015 + 1.001x. 0.925 = 1.001x, x = 0.924, so 92.4% ≈ 92.5%.
Hint
Set up an equation where the average atomic mass is the weighted sum of isotopic masses.