Answer
:
83.9 u
Explanation:
The atomic mass of Kr is the <em>weighted average</em> of the atomic masses of its isotopes.
We multiply the atomic mass of each isotope by a number representing its <em>relative importance</em> (i.e., its fraction of the total).
1. Use the mass spectrum to calculate the <em>fraction of each isotope</em>.
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. <em>Set up a table</em> 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 = <u>15.2 u
</u>
TOTAL = 83.9 u
The actual atomic mass of Kr is 83.8 u.
<em>Note</em>: The calculated value above is incorrect because the relative abundances in the diagram are not quite correct.