Question

Rubidium ignites spontaneously when exposed to oxygen to for rubidium
Rubidium ignites spontaneously when exposed to oxygen to for rubidium oxide. Rb²0. Rubidium exists as two isotopes (84.91u) and (86.91u). if the average atomic mass of rubidium is (84.91u), determine the percentage abundance of 85/37Rb

Answer 1

Answer on Question #44032 - Chemistry - Inorganic Chemistry

Question:

Rubidium ignites spontaneously when exposed to oxygen to for rubidium oxide. Rb₂O. Rubidium exists as two isotopes (84.91u) and (86.91u). If the average atomic mass of rubidium is (84.91u), determine the percentage abundance of 85/37Rb

**Note:** If the average atomic mass of rubidium is (84.91u), how it is written in the task, then

The abundance of isotope #1 (84.91u) = 100%

The abundance of isotope #2 (86.91u) = 0%

But maybe it's a mistake. That's why we will calculate the abundance for average atomic mass of rubidium = 85 (in the task 85/37Rb).

Solution:

average atomic weight of the element = (exact weight of isotope #1) · (abundance of isotope #1) + (exact weight of isotope #2) · (abundance of isotope #2)

We have to remember that abundance of isotope #1 + abundance of isotope #2 = 1

Then

abundance of isotope #1 = x

abundance of isotope #2 = 1 - x

Let's put all the meanings in our equation:

average atomic weight of the element = (exact weight of isotope #1) · (abundance of isotope #1) + (exact weight of isotope #2) · (abundance of isotope #2)

$$85 = 84.91 \cdot x + 86.91 \cdot (1 - x)$$ $$85 = 84.91x + 86.91 - 86.91x$$ $$2x = 1.91$$ $$X = 0.955$$

The abundance of isotope #1 (84.91u) = 95.5%

The abundance of isotope #2 (86.91u) = 100% - 95.5% = 4.5%

**Answer:** The abundance of isotope #1 (84.91u) = 95.5%

The abundance of isotope #2 (86.91u) = 4.5%

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