Question

Calculate the average atomic mass of rubidium. Rubidium has two isotopes, 85Rb and 87Rb. 85Rb has an atomic mass of 84.912 amu and occurs at an abundance of 72.17%. 87Rb has an atomic mass of 86.909 amu and occurs at an abundance of 27.83%. Show your work.

Answer 1

Answer : The average atomic mass of rubidium = 85.468 Explanation: Given that 85Rb isotope has atomic mass of 84.912amu with abundance of 72.17% therefore we get 84.912 x 0.7217 = 61.281 and 87Rb isotope has atomic mass of 86.909 with abundance of 27.83% hence we get 86.909 x 0.2783 = 24.187 Thus we get 61.281 + 24.187 = 85.468 as the average atomic mass of Rubidium.

Answer 2

The average atomic mass of rubidium is 85.466 amu.

For calculating the average atomic mass of the Rb, the atomic masses for both the isotopes are taken into consideration.

The $\rm ^8^5Rb$ has atomic mass = 84.912 amu

The abundance of $\rm ^8^5Rb$ = 72.17 %

The average atomic mass of $\rm ^8^5Rb$ = 84.912 $\rm \times\;\dfrac{72.17}{100}$

= 61.28 amu

The $\rm ^8^7Rb$ has atomic mass = 86.909 amu

The abundance of $\rm ^8^7Rb$ = 27.83%

The average atomic mass of $\rm ^8^7Rb$ = 86.909 $\rm \times\;\dfrac{27.83}{100}$

= 24.186 amu

The average atomic mass of Rb will be = $\rm ^8^5Rb$ + $\rm ^8^7Rb$

= 61.28 + 24.186 amu

= 85.466 amu.

The average atomic mass of rubidium is 85.466 amu.

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