Question

Elemental magnesium consists of three isotopes: 24Mg with an accurate mass of 23.99 amu, 2 Mg with an accurate mass of 24.99 amu, and 26Mg with an accurate mass of 25.99 amu. If 24Mg is 79% of naturally occurring magnesium, what is the percent abundance of 26Mg?

Answer 1

Given : Elemental magnesium consists of three isotopes: 24Mg with an accurate mass of 23.99 amu, 25 Mg with an accurate mass of 24.99 amu, and 26Mg with an accurate mass of 25.99 amu.

To find : If 24Mg is 79% of naturally occurring magnesium, the percent abundance of 26Mg is ...

solution : experimentally, the average atomic mass of Mg = 24.31 amu

let percentage abundance of 26 Mg is x %

so percentage abundance of 25 Mg is (100 - 79 - x) = (21 - x) %

now average atomic mass = sum of % abundance × accurate mass

⇒24.31 = 79% of 23.99 amu + x % of 25.99 + (21 - x)% of 24.99

⇒24.31 = {79 × 23.99 + 25.99 x + 24.99(21 - x)}/100

⇒2431 = 1895.21 + 25.99x + 524.79 - 24.99x

⇒2431 = 2420 + x

⇒x = 11 %

Therefore the percentage abundance of 26 Mg is 11 %