# Calculating Average Atomic Mass

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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 -

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 -

85+87
2

• Calculating Average Atomic Mass -

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 -

58+87

• Calculating Average Atomicc Mass -

the terms"average atomic mass is............ Average,and
so is calculate different from a "normal" Average?

• Calculating Average Atomic Mass -

488.3 repeating