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(a) Two stable isotopes of lithium " "(3...

(a) 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 6.01512u and 7.01600u, respectively. Find the atomic mass of lithium.
(b) Boron has two stable isotopes, `" "_(5)^(10)B` and `" "_(5)^(11)B`. Their respective massis are 10.01294u and 11.00931u, and the atomic mass of boron is 10.811 u. Find the abundances of `" "_(5)^(10)B` and `" "_(5)^(11)B`.

Text Solution

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(a) Average atomic mass of lithium is the weighted average of masses of two isotopes `" "_(3)^(6)Li` and `" "_(3)^(7)Li`, =`(7.5xx6.01512+92.5xx7.0160)/100`u=6.9409u`approx`6.941 u.
(b) It is given here that mass of `" "_(5)^(10)B` nuclide is 10.01294 u and that of `" "_(5)^(11)B` nuclide is 11.00931 u. Let abundance of `" "_(5)^(10)B` in boron be x% and abundance of `" "_(5)^(11)B` be (100-x)%. Then Atomic mass of boron 10.811=`(10.01294xxx+11.00931xx(100-x))/100 implies x=19.9%`
Hence, abundance of `" "_(5)^(10)B` nuclide in boron is 19.9% and abundance of `" "_(5)^(11)B` isotopes is (100-19.9%)=80.1%.
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