Answer:
Average Atomic Weight (X) = 87.638974 amu
Step-by-step explanation:
To find the average atomic weight of an unknown element that exhibit multiple isotopes is given by:
Average Atomic Weight = Percentage of given isotope in the mass spectrum x The corresponding atomic weight of given isotope
Average Atomic Weight (X) = [0.0056 x 83.91343] + [0.098860 x 85.90927] + [0.07 x 86.90890] + [0.8258 x 87.90562] = 87.638974 amu