Answer :
83.9 u
Explanation:
The atomic mass of Kr is the weighted average of the atomic masses of its isotopes.
We multiply the atomic mass of each isotope by a number representing its relative importance (i.e., its fraction of the total).
1. Use the mass spectrum to calculate the fraction of each isotope.
Total relative abundance = 2.0 + 2.0 + 10.0 + 3.0 = 17.0
Fraction of Kr-82 = 2.0/17.0
Fraction of Kr-83 = 2.0/17.0
Fraction of Kr-84 = 10.0/17.0
Fraction of Kr-86 = 3.0/17.0
2. Set up a table for easy calculation
2.0/17.0 × 81.9 u = 9.64 u
2.0/17.0 × 82.9 u = 9.75 u
10.0/17.0 × 83.9 u = 49.35 u
3.0/17.0 × 85.9 u = 15.2 u
TOTAL = 83.9 u
The actual atomic mass of Kr is 83.8 u.
Note: The calculated value above is incorrect because the relative abundances in the diagram are not quite correct.