Average atomic mass = f1M1 + f2M2 +… + fnMn
where f is the fraction representing the natural abundance of the isotope and M is the mass number (weight) of the isotope.
let x be the fraction of ^6li ,
then fraction abundance of [tex]^7li[/tex] will be (1-x)
now applying above equation:
6.941 = x × 6.0151 +(1-x) 7.0160
1.0009x = 0.075
x = 0.0749
so % of [tex]^6li[/tex]= 7.49%
% of [tex]^7li[/tex]= 92.50%
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