Calculate the average atomic mass of rubidium. rubidium has two isotopes, & rb and 7rb. 85rb has an atomic mass of 84.912 amu and occurs at an abundance of 72.17% 87rb has an atomic mass of 86.909 amu and occurs at an abundance of 27.83% show your work.

The average atomic mass of Rb is 85.47 u.  

The average atomic mass of X 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 % abundance).  

Thus,  

0.7217 × 84.912   u =  61.282 u  

0.2783 × 86.909 u = 24.187  u  

                   TOTAL = 85.47   u  

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