Two stable isotopes of lithium `._3^6Li` and `._3^7Li` have respective abundances of 7.5% and 92.5% . These isotopes have masses 6.0152 u and 7.016004 u respectively. Find the atomic weight of lithium

Explanation

Abundance of `""_(3)^(b)Li` is `7.5%` and abundance of `""_(3)^(7)Li` is `92.5%`
hence atomic mass of lilthium
`=(7.5(6.01512u)+92.5(7.01600u))/(100)`
`A=(45.1134+648.98)/(100)u`
`A=6.941u`
(b) Let abundance of `""_(5)^(10)B` is `x%` then abundance of `""_(5)^(11)B` will be `(100-x)%`
Atomic mass of boron
`=(x[10.01294u]+(100-x)[11.00931u])/(100)`
`100xx10.811u=1100.931u-0.99637x u`
`"Solving we get, "x=(19.831)/(0.99637)=19.9`
`"So, relative abundance of "_(5)^(10)B" isotope "=19.9%`
`"Relative abundance of "_(5)^(11)B" isotope "=80.1%`