What is the Average Atomic Mass of lithium?

What is the Average Atomic Mass of lithium if 7.42% exists as 6Li (6.015 amu) and 92.58% exists as 7Li (7.016 amu)?

I'm having trouble. Thank you :)


You need to find the weighted average. You can also find it on a periodic table :)

6.015*.0742 + 7.016*.9258 = approximately 6.942

On the periodic table, the atomic mass is 6.941, so that is pretty close :)


The average atomic mass is a weighted average, which means that it must take into account the relative abundance of each isotope.

(6.015 amu x 0.0742) + (7.016 amu x 0.9258) = 6.9417 amu