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Question

Mass numbers of two isotopes of an element differ by 2 units (A and A+2). Average atomic mass is 0.5 more than the lower mass number. What could be the ratio of the percentage abundance two isotopes respectively?

A
3 : 1
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B
1 : 2
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C
1 : 4
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D
1 : 1
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Solution

The correct option is A 3 : 1According to given conditions,Mass number of isotope of lower mass number be A.Mass number of 2nd isotope =(A+2)Average atomic mass =(A+0.5)If x is the percentage abundance of lower mass number isotope, then (100−x) is the percentage abundance of 2nd isotope.Since (A+0.5)=a1x1+a2x2(x1+x2)=x(A)+(100−x)(A+2)100100A+50=xA+(100−x)(A+2)=xA+100A+200−xA−2x2x=150;x=75100−x=25so their ratio =3:1

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