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Question
Two stable isotopes of lithium `""_3^6"Li"` and `""_3^7"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.
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Solution
Mass of lithium isotope `""_3^6 "Li"`, m1 = 6.01512 u
Mass of lithium isotope `""_3^7 "Li"`, m2 = 7.01600 u
Abundance of `""_3^6 "Li"`, η1 = 7.5%
Abundance of `""_3^7 "Li"`, η2 = 92.5%
The atomic mass of lithium atom is given as:
`"m" = ("m"_1η_1 + "m"_2η_2)/(η_1 + η_2)`
= `(6.01512 xx 7.5 + 7.01600 xx 92.5)/(92.5 + 7.5)`
= 6.940934 u
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