Question

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.

Answer 1

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