Question

Rubidium is a soft, silvery-white metal that has two common isotopes, `""^85 Rb` and `""^87 Rb`. If the abundance of `""^85 Rb` is 80.2% and the abundance of `""^87 Rb` is 19.8%, what is the average atomic mass of rubidium,?

Answer 1

The average atomic mass should be the WEIGHTED AVERAGE of the individual isotopes.

Explanation:

So, we take the weighted atomic mass of `""^87Rb` and `""^85Rb`.

`(85xx80.2% + 87xx19.8%)"amu" = 85.40` `"amu"`. Check to see if this atomic mass is close to that listed on the Periodic Table, which of course considers ALL the isotopes,