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 (23.98504u), (24.98584u) and (25.98259u). The natural abundance of is 78.99% by mass. Calculate the abundances of other two isotopes.

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Solution

Average atomic mass of magnesium, m = 24.312 u

Mass of magnesium isotope, m₁ = 23.98504 u

Mass of magnesium isotope, m₂= 24.98584 u

Mass of magnesium isotope, m₃= 25.98259 u

Abundance of, η₁= 78.99%

Abundance of, η₂= x%

Hence, abundance of, η₃= 100 − x − 78.99% = (21.01 − x)%

We have the relation for the average atomic mass as:

Hence, the abundance of is 9.3% and that of is 11.71%.

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