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## QuestionPhysicsClass 12

Two stable isotopes of lithium $$^6 3\mathit{Li}$$ and $$^73\mathit{li}$$ have respective abundances of $$7.5\%$$ and $$92.5\%$$.These isotopes have masses $$6.01512 u$$and $$7.01600 u$$, respectively. Find the atomic mass of lithium.

Mass of $$^6 3\mathit{Li}$$,$$m_1=6.01512u$$
Mass of $$^73\mathit{Li}$$, $$m_2=7.01600u$$
Abundances of the following are $$n_1=7.5%$$ and $$n_2=92.5%$$
Atomic mass of $$Li$$ is given by= $$\frac{m_1n_1+m_1n_1}{m_1+m_2}$$
= $$\frac{6.01512\times 7.5+7.01600\times 92.5}{6.01512+7.01600}$$
The atomic number of $$Li$$ is $$6.9409344u$$.