1. Rubidium occurs naturally as a mixture of two isotopes, 72.17% Rb-85 (atomic mass 84.911 792 u) and 27.83% Rb-87 (atomic mass 86.909 186 u).
What is the average atomic mass of rubidium?
2. The element silicon occurs as a mixture of three isotopes: 92.22% Si-28, 4.69%
Si-29, and 3.09% Si-30. The atomic masses of these three isotopes are as follows:
Si-28 = 27.976 926 u, Si-29 = 28.976 495 u, and Si-30 = 29.973 770 u.
Find the average atomic mass of silicon.