The element rubidium has two naturally occurring isotopes. The atomic mass of 85Rb (72.17 percent abundant) is 84.911794 amu. Determine the atomic mass of 87Rb (27.83 percent abundant). The average atomic mass of Rb is 85.4678 amu.

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