Question

An element A has the following isotopic composition 12A : 40%, 15A : 60%. The average atomic mass of the element A is Choose answer:
13.8 u
14.5 u
12.7 u
13 u

Answer 1

The average atomic mass of the element A is equal to 13.8 u.

n1 - the percentage of 12A isotope of the element A = 40%

n2 - the percentage of 15A isotope of the element A = 60%

X1 - the mass of the 12A isotope = 12

X2 - the mass of the 15A isotope = 15

The average atomic mass of the element A = mean of the mass of the two isotopes of the element A

= (n1X1 + n2X2)/(X1 + X2)

= (40×12 + 60×15)/100

= 13.8 u