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**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:

$\overline{){\mathbf{Atomic}}{\mathbf{}}{\mathbf{Mass}}{\mathbf{=}}{{\mathbf{[}}{\mathbf{mass}}{\mathbf{\times}}{\mathbf{f}}{\mathbf{.}}{\mathbf{a}}{\mathbf{.}}{\mathbf{]}}}_{\mathbf{isotope}\mathbf{}\mathbf{1}}{\mathbf{+}}{{\mathbf{[}}{\mathbf{mass}}{\mathbf{\times}}{\mathbf{f}}{\mathbf{.}}{\mathbf{a}}{\mathbf{.}}{\mathbf{]}}}_{\mathbf{isotope}\mathbf{}\mathbf{2}}}$

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 ** ** **

###### 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

Frequently Asked Questions

What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Atomic Mass concept. You can view video lessons to learn Atomic Mass Or if you need more Atomic Mass practice, you can also practice Atomic Mass practice problems .