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  4. Two stable isotopes 63Li and 73Li have respective abundances of 7.5 % and 92.5 %. These isotopes have masses 6.01512 u and 7.01600 u respectively. The atomic weight of lithium is

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Q. Two stable isotopes $ ^{6}_{3}Li$ and $ ^{7}_{3}Li$ have respective abundances of $7.5\%$ and $92.5\%$. These isotopes have masses $6.01512\,u$ and $7.01600\,u$ respectively. The atomic weight of lithium is

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Solution:

Atomic weight = average weight of the isotopes
$=\frac{6.01512\times7.5+7.01600\times92.5}{\left(7.5+92.5\right)}=\frac{45.1134+648.98}{100}$
$=6.941\,u$