1.

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 -

Answer»

`8%`
`10%`
`2%`
`0.5%`

Solution :`{:(,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%`


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