Question

PLEASE HELP!! In a sample of 400 lithium atoms, it is found that 30 atoms are lithium-6 and 370 atoms are lithium-7. Calculate the average atomic mass of lithium.

Answer 1

isotopic mass of lithium 6 is 6.015, and for 7 is 7.016(Wikipedia).
now % of abundance of 6 isomer is equal to 30/400=7.5%,
% of abundance of 7 isomer is equal to 370/400=92.5%
so av atomic mass= ((mass of the 1st isotope * relative abundance )+(mass of 2nd isotope*relative abundance))
so av mass =((6.015*0.075)+(7.016*.925)
=6.9409