Question

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

Answer 1

To try calculate the atomic masses of our molecules we define calculate the relative (average) atomic masses. The relative/average comes from the idea that we weight the masses with their probability of occurrence. In terms of math:
\[\large M_{element}=\sum_i m_i \times p_i\]
Here is \(m_i\) the mass of the \(i\)'th isotope with the probability of occurrence \(p_i\).

If the math confuses you, I can show you more specifically how to apply this for rubidium.

Answer 2

Yeah if you don't mind showing the math to me that would be really helpful

Answer 3

Basically the function just say we must add up these "weighted masses"

So:
\[\large M_{ribidium}=m(^{85}\text{Rb}) \times p(^{85}\text{Rb}) +m(^{87}\text{Rb}) \times p(^{87}\text{Rb})\]

From here it is all about plugging in numbers, just remember to make your percentages into decimal numbers.

Answer 4

Thank you so much for your help! :)

Answer 5

No problem at all. If you want to write out your answer I can check it for you before you turn it in.