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 2412Mg (23.98504u), 2512Mg(24.98584u) and 2612Mg (25.98259u). The natural abundance of 2412Mg is 78.99% by mass. Calculate the abundances of the other two isotopes.
Let, the abundance of 12Mg25 by mass be x%. Therefore, abundance of 12Mg26 by mass=(100−78.99−x)%=(21.01−x)% Now, average atomic mass of Mg is 24.312=23.98504×10078.99+24.98584×100x+25.98259×10021.01−x ⟹x=9.303% for 12Mg25 (21.01−x)% =11.71% for 12Mg26
