Can you assist me with this, been stuck for ages

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.