Chemistry question =----

Question

Rubidium has two naturally occurring isotopes. The weight average atomic mass of Rb is 85.4678 u. If 72.15% of Rb is found as Rb-85 (84.9117 u), what is the mass of the other isotope?

a. 0.5657 u

b. 85.6877 u

c. 87.6890 u

d. 86.0295 u

e. 86.9085 u

Answer 1

The answer to this question is E. This is because we are doing a weighted average. Let us imagine that of the known 85.4678 u average weight, we know that 72.15% is from the isotope with the mass of 84.9117 u, and 27.85% is from the isotope with the unknown mass, X. We can say then that 0.7215*84.9117 + 0.2785* X = 85.4678 u. We then can solve for X which happens to be 86.9085 which is option E. We also know that this answer makes sense because the first mass was smaller than the average, so the second mass must be bigger than the average to raise the first one. I hope this helps!