What is the atomic mass of #""^6Li#, if the atomic mass of lithium is 6.941 amu, and #""^7Li# is 92.58% abundant and weighs 7.015 amu?

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Answer 1 · 2016-12-17T16:43:05

#"Mass of "^6Li" isotope"=(0.446*"amu")/(0.0742)=6.011*"amu"#

The quoted atomic mass is the weighted average of the individual isotopic masses of #""^7Li# and #""^6Li#.

#6.941# #"amu"# #=# #7.015*"amu"xx92.58%+("mass of "^6Li" isotope")xx7.42%#

And thus, #"6.495 amu"+("mass of "^6Li" isotope")xx7.42%=6.941*"amu"#

And thus,

#("mass of "^6Li" isotope")xx7.42%=(6.941-6.495)*"amu"#

#("mass of "^6Li" isotope")xx7.42%=0.446*"amu"#

#"mass of "^6Li" isotope"=(0.446*"amu")/(0.0742)=6.011*"amu"#.

How did I know that #""^6Li# had an abundance of #7.42%#?