Problem: Rubidium has two isotopes, rubidium-85 and rubidium-87. The atomic weight of rubidium is 85.4678 amu. If the atomic weight of rubidium-85 is 84.9118 amu which is present as 72.15% of all rubidium isotopes, what is the atomic weight of rubidium-87?A) 86.9082 amuB) 87.0054 amuC) 86.0000 amuD) 86.9563 amuE) 87.1123 amu

FREE Expert Solution

We’re being asked to determine the atomic weight of rubidium-87.


We can use the following equation:


Atomic Mass=[mass×f.a.]isotope 1+[mass×f.a.]isotope 2


where:

atomic mass = average atomic mass of the element 

f.a. = fractional abundance of the isotope

To get f.a., we simply need to divide the given percent abundance by 100.


We’re given the following values:

Average Atomic Mass = 85.4678 amu

Rubidium -85: Mass = 84.9118 amu

                        f.a. = 0.7215            

Rubidium -87: Mass = ????

We don’t know the percent abundance of 1 isotope but we can use the fact that the f.a. of all isotopes of an element add up to 1

                        f.a. = (1 - 0.7215) = 0.2785   


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Problem Details

Rubidium has two isotopes, rubidium-85 and rubidium-87. The atomic weight of rubidium is 85.4678 amu. If the atomic weight of rubidium-85 is 84.9118 amu which is present as 72.15% of all rubidium isotopes, what is the atomic weight of rubidium-87?

A) 86.9082 amu

B) 87.0054 amu

C) 86.0000 amu

D) 86.9563 amu

E) 87.1123 amu

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