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NCERT Class XII Chapter
Nuclei
Questions With Solutions

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Question : 23
Total: 31
In a periodic table the average atomic mass of magnesium is given as 24.312 u. The average value is based on their relative natural abundance on earth. The three isotopes and their masses are
24
‌
12
Mg (23.98504u),
25
‌
12
Mg (24.98584 u) and
26
‌
12
Mg (25.98259u). The natural abundance of
24
‌
12
Mg is 78.99% by mass.
Calculate the abundances of the other two isotopes.
Solution:  
Let the abundance of isotope
26
‌
12
Mg is x%, then the abundance of isotope
25
‌
12
Mg is [100 – (x + 78.99)]%.
Average atomic mass of Mg 24.312 =
78.99×23.98504+[100−(x+78.99)]24.98584+x[25.98259]
100

24.312 = 1894.5783 + 2498.564 + 0.99675 x – 1973.6157
x =
11.6734
0.996875
 = 11.71
so, x = 11.71%, thus the abundance of
26
‌
12
Mg is 11.71% and of
25
‌
12
Mg is 9.303%.
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