Question

the average atomic mass of a sample of an element x is 16.2 u. what is the percentage of isotopes X 8,16 and X 8,18 in the sample.

Answer 1

Answer:

let atomic mass a = X ( 8,16 )

let atomic mass b = X ( 8,18 )

percentage of atomic mass a = x %

percentage of  atomic mass b = ( 100 - x ) %

avg atomic mass = % atomic mass a + % atomic mass b

16.2 = 16 * x / 100 + 18 * ( 100 - x ) / 100

16.2 = 16x / 100 + 1800 - 18x / 100

16.2 = 16x / 100 - 18x / 100 + 1800

16.2 = -2x / 100 + 1800

1620 = - 2x + 1800

1620 - 1800 = -2x

-180 = -2x

x = -180 / -2

x = 90 %

(by substituting the values for x) :

percentage of atomic mass a = x %                = 90 %

percentage of  atomic mass b = ( 100 - x ) %  = 100 - 90 = 10 %