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The atomic masses of 6Li and 7Li are 6.0151amu and 7.0160 amu , respectively. Calculate the natural abundances of these two isotopes. The average atomic mass of Li is 6.941

The sum of natural abundances of isotopes should equal = 1
If the natural abundance of isotope 1 = x1
then the natural abundance of isotope 2 = x2
So x1 + x2 = 1 → x2 = 1- x1
Average atomic mass =(natural abundance × atomic mass) isotope 1+ (natural abundance × atomic mass) isotope 2
6.941 = 6.0151× x1 + 7.0160 × (1- x1 ) = 6.0151x1 + 7.0160 - 7.0160 x1
-7.0160 + 6.941 = (6.0151x1 - 7.0160 x1) = 0.1 = x
→ x1 = 7.5 5 → x2 = 1- x1