## anonymous one year ago 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.

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1. Frostbite

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.

2. anonymous

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

3. Frostbite

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.

4. anonymous

Thank you so much for your help! :)

5. Frostbite

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