Two stable isotopes ${ }_3^6 \mathrm{Li}$ and ${ }_3^7 \mathrm{Li}$ have respective abundances of $7

Question

Two stable isotopes ${ }_3^6 \mathrm{Li}$ and ${ }_3^7 \mathrm{Li}$ 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

Professor PC Thomas Classes · Answer

(a) : Atomic weight $=$ average weight of the isotopes
$=\frac{6.01512 	imes 7.5+7.01600 	imes 92.5}{(7.5+92.5)}$\ $=\frac{45.1134+648.98}{100}$\ $=6.941 \mathrm{u}$