Rubidium is a soft, silvery-white metal that has two common isotopes, 85Rb and 87Rb. If the abundance of 85Rb is 72.2% and the abundance of 87Rb is 27.8%, what is the average atomic mass of rubidium?
1 Answer
Explanation:
As you know, the average atomic mass of an element is calculated by taking the weighted average of the atomic masses of its stable isotopes.
More specifically, the average atomic mass of an element is calculated by taking into account the atomic masses of its stable isotopes and their decimal abundance, which is imply the percent abundance divided by
avg. atomic mass=∑i(isotopei×abundancei)
So, you know that rubidium has two stable isotopes,
When the problem doesn't provide the atomic masses of the stable isotopes, you can use their mass numbers. As you know, an isotope's mass number tells you how many protons and neutrons it contains in its nucleus.
In this case,
The average atomic mass of rubidium will thus be
avg. atomic mass=85 u×0.722+87 u×0.278
avg. atomic mass = 85.556 u
Rounded to three sig figs, the number of sig figs you have for the percent abundances of the two isotopes, the answer will be
avg. atomic mass = 85.6 u
