Question

An unknown element has 2 naturally occurring isotopes. The first isotope has an atomic mass of 84.9117 and the second isotope has an atomic mass of 86.909. Their abundances are 72.2% and 27.8% respectively.
Answer Choices: 85.56 86.44 85.21 86.00

Answer 1

Answer:The other isotope, 65Cu, has an abundance of 30.91%. Calculate the average atomic mass of copper. 63Cu 0.6909 x 63= 43.5267 65Cu 0.3091 x 65= +20.0915

Explanation:Nov 17, 2015 — The average atomic mass of an element is determined by taking the weighted average of the atomic masses of its naturally occurring isotopes. Now, weighted average ... avg. atomic mass=∑i(isotopei×abundancei). The more ... of the element. In your case, you know that rubidium has two stable isotopes.

Answer 2

Explanation:

To find the average atomic mass of the element we must take a weighted average. Using the formula $am = (m_{1} * abundance)$  . Plug and chug gives us

$am = (84.9117 * 0.722) + (86.909 * 0.278) = approx. 85.56g/mol$

The element has an average mass of 85.56 amu.