Answer: 85.47u
Explanation:
The average atomic mass of an element is determined by taking the weighted average of the atomic masses of its naturally occurring isotopes.
Now, weighted average simply means that each isotope contributes to the average atomic mass of the element proportionally to its percent abundance.
avg. atomic mass=∑i(isotopei×abundancei)
The more abundant an isotope is, the more its atomic mass will influence the average atomic mass of the element.
In your case, you know that rubidium has two stable isotopes
85Rb → 84.91 u, 72.16% percent abundance
87Rb →86.91 u, 27.84% percent abundance
When you calculate the average atomic mass, make sure that you use decimal abundance, which is simply percent abundance divided by
100.
So, plug in your values to get
avg. atomic mass = 84.91 u × 0.7216 + 86.91 u × 0.2784
avg. atomic mass = 85.4668 u
Rounded to four sig figs, the answer will be
avg. atomic mass =85.47 u
