Question

The mass contributions of the two isotopes are:


61.27 u Rb-85


24.20 u Rb-87


What is the average atomic weight?

Answer 1

average atomic weight = $\sum$ atomic weight of the isotopes * mass contributions of the isotopes

According to the conditions of the problem, average atomic weight of Rb = 85*0.6127 + 87*0.2420 = 73.13 u.

But these values of mass contributions are wrong! The correct values (taken from Wiki) are 72.2% for Rb-85 and 27.8% for Rb-87. Use these values for calculations:

Average atomic weight of Rb = 85*0.722 + 87*0.278 = 85.56 u.