Question

An element A has three isotopes `A^(20), A^(21), A^(22)`. The % abundance of `A^(20)` is 90% by mole and average atomic mass of the element A is 20.18 then mole % abundance of `A^(21)` would be -
A. `8%`
B. `10%`
C. `2%`
D. `0.5%`

Answer 1

Correct Answer - C
`{:(,A^(20),A^(21),A^(22),M_(avg) = 20.18,),(,darr,darr,darr,,),(% "by moles",90%,(10 - x)%,x%,,):}`
`M_(avg) = sum %` of isotope `xx` atomic mass
`20.18 = (90 xx 20 + (10 - x) 21 + x xx 22)/(100)`
`2018 = 1800 + 210 - 21 x + 22 x`
`2018 = 2018 + x`
`x = 8 %`
`%` abundance of `.^(21)A = (10 - x) = 2%`