Rubidium possesses two stable forms and has an average mass of 85.5.⁸⁵Rb has a mass of 84.9 and a percent abundance of 72.2%.What is the mass of the other form of Rb? A)86.1 u B)85.7 u C)88.3 u D)87.1 u E)89.4 u

Question

Quizplus · Accepted Answer

Let x be the mass of the other form of Rb.
The average mass of rubidium can be expressed using the percent abundance and masses of the two forms:
85.5 = 0.722(84.9) + 0.278(x)
Solving for x:
x = (85.5 - 0.722(84.9))/0.278
x = 87.1
Therefore, the mass of the other form of Rb is 87.1 u, and the answer is D).

Rubidium Possesses Two Stable Forms and Has an Average Mass