Posted by **Lena** on Saturday, September 8, 2007 at 11:57pm.

Rubidium has 2 common isotopes, Rb 85 and Rb 87. If the abudance of Rb 85 is 70 percent and the abundance of Rb 87 is 30 percent, what is the average atmoic mass of rubidium?

For the above question, does this mean I would multiple 85 by .70, 87 by .30, and average those two answers? Thank you!

- Calculating Average Atomic Mass -
**drwls**, Sunday, September 9, 2007 at 5:48am
Yes; however the answer you get will not be exact since the atomic masses of isotopes 85 and 87 are not exactly 85 and 87. There are small differences due to binding energy (Relativity effects).

- Calculating Average Atomic Mass -
**sul**, Monday, September 24, 2007 at 12:32pm
85+87

2

- Calculating Average Atomic Mass -
**RiOtKiDd**, Friday, October 3, 2008 at 9:43am
Well i thought to calculate the average atomic mass, you would have to turn the percentages into a decimal and multiply 85 by .70 and 87 by .30 and add them together, there is no need to divide.

- Calculating Average Atomicc Mass -
**Anonymous**, Thursday, December 6, 2012 at 5:11pm
58+87

- Calculating Average Atomicc Mass -
**asiya**, Thursday, December 6, 2012 at 5:17pm
the terms"average atomic mass is............ Average,and

so is calculate different from a "normal" Average?

- Calculating Average Atomic Mass -
**Tyler**, Tuesday, March 19, 2013 at 6:00pm
488.3 repeating

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