Question

Naturally occuring lithium consists of two isotopes whose atomic weights are 6.015 u and 7.016 u. The atomic weight of natural lithium is 6.491 u. The percentage by number of atoms of each isotpe in natural lithium is and respectively.

Answer 1

Let the fraction of Li-6.015 be x.
So, the fraction of Li-7.016 will be 1-x.
According to the question,
$60.015x+7.016(1-x)=6.941$
$\Rightarrow 1.001x=0.075\Rightarrow x=0.07493$
So, the percentage of Li-6.015 is 7.493% and the percentage of Li-7.016 is 92.507%