Answer
The average atomic mass of Rb = 85.6 amu
Explanation
Abundance of ⁸⁵Rb = 72.2% = 0.722
Atomic mass of ⁸⁵Rb = 85
Abundance of ⁸⁷Rb = 27.8% = 0.278
Atomic mass of ⁸⁷Rb = 87
Average atomic mass = ?
First, the exact atomic mass of each isotope must be obtained. Then multiply the percent abundance as a decimal for both isotopes and add them together i.e
Average atomic mass of Rb = (abundance of ⁸⁵Rb × its atomic mass) +(abundance of ⁸⁷Rb × its atomic mass) / 100
[tex]\begin{gathered} \text{Average atomic mass of Rb }=\frac{(72.2\times85)+27.8\times87}{100} \\ =\frac{6137+2418.6}{100}=\frac{8555.6}{100}=85.556\text{ amu} \\ \text{Average atomic mass of Rb }\approx85.6\text{ amu} \end{gathered}[/tex]The average atomic mass of rubidium is 85.6 amu
