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.

Answer 1

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