Find the relative atomic mass of 25Mg

Question

Naturally occurring magnesium has an atomic mass of 24.312 and consists of three isotopes. The major isotope is 24Mg, natural abundance 78.99%, relative atomic mass 23.98504. The next most abundant isotope is 26Mg, relative atomic mass 25.98259. The third most abundant isotope is 25Mg, whose natural abundance is in the ratio of 0.9083 to that of 26Mg. Find the relative atomic mass of 25Mg.

Answer 1

Since 24Mg represents 78.99% abundance, the total abundance represented by the other two isotopes is 100 - 78.99 = 21.01%

If x = % represented by 26Mg, then 0.9083x = that for 25Mg

X + 0.9083 x + 0.7899 = 1.0000

1.9083x + 0.7899 = 1.0000

x = 0.1101 = abundance of 26Mg

0.9083x = 0.1000 = abundance of 25Mg

(0.7899)(23.98504) + (0.1101)(25.98259) + (0.1000)(a) = 24.312

18.9458 + 2.8607 + 0.1000a = 24.312

0.1000a = 2.5055

a = atomic mass of 25Mg = 25.055