Answer
Atomic mass of Strontium = $ 87.6166amu$
Work Step by Step
Mass of Isotope 1 = $83.9134 amu $
% abundance of Isotope 1 = $0.56\%$
Mass of Isotope 2 = $85.9094 amu$
% abundance of Isotope 2 = $9.86\%$
Mass of Isotope 3 = $86.9089 amu$
% abundance of Isotope 3 = $7.00\%$
Mass of Isotope 4 = $87.9056 amu$
% abundance of Isotope 4 = $82.58\%$
Atomic mass of an element is the weighted average of the masses of its constituent isotopes.
We convert the % abundance of each Isotope into fractions. Hence,
Atomic mass of Strontium = $0.0056(83.9134amu)+0.0986(85.9094amu)+0.0700(86.9089amu)+0.8258(87.9056amu)$
Atomic mass of Strontium = $87.6166amu$
