An element with relative atomic mass 20.8 is found to contain 2 isotopes having a relative atomic mass of 20 and 21.Find the % composition of the element.

Answer 1

Relative atomic mass = 20.8 g

Let the fraction that is element 20 = x
thus the fraction that is element 21=1-x

so 20x +21(1-x) = 20.8

20x + 21 -21x=20.8

-1x=-0.2

x=0.2 and 1-x=0.8

so 20% element 20 and 80% element 21.

Answer 2

20.8%