24 25
21. Let f24 be the abundance of Mg, let f25 be the abundance of Mg, and let f26 be the abundance of
26
Mg. Then, the entry in the periodic table for Mg is 24.312 = 23.98504f24 +24.98584f25 +25.98259f26.
Since there are only three isotopes, f24 + f25 + f26 =1. We solve for f25 and f26. The second equation
gives f26 =1- f24 - f25. We substitute this expression and f24 =0.7899 into the first equation to obtain
24.312 = (23.98504)(0.7899) + 24.98584f25 +25.98259 - (25.98259)(0.7899) - 25.98259f25. The solution
is f25 =0.09303. Then, f26 =1 - 0.7899 - 0.09303 = 0.1171. 78.99% of naturally occurring magnesium
24 25 26
is Mg, 9.30% is Mg, and 11.71% is Mg.

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