# 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?

Dec 17, 2016

$\text{Mass of "^6Li" isotope"=(0.446*"amu")/(0.0742)=6.011*"amu}$

#### Explanation:

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

$6.941$ $\text{amu}$ $=$ 7.015*"amu"xx92.58%+("mass of "^6Li" isotope")xx7.42%

And thus, $\text{6.495 amu"+("mass of "^6Li" isotope")xx7.42%=6.941*"amu}$

And thus,

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

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

$\text{mass of "^6Li" isotope"=(0.446*"amu")/(0.0742)=6.011*"amu}$.

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