Question

EXERCISE
21.
Atomic weight of chlorine is 35.5. It has two isotopes of atomic weight 35 and 37. What is the percentage
of the heavier isotope in the sample?
(1) 5
(2)10
(3)25
(4)20​

Answer 1

given :

Atomic Mass of Chlorine = 35.5

let the first isotpe Cl- 35 be x% and second isotope Cl-37 be (100-x)%

=> x/100 × 35 + (100-x)/100 × 37 = 35.5

=> 35x/100 + (3700-37x)/100 = 35.5

=> 3700-37x+35x = 35.5 ×100

=>3700-2x = 3550

=> 2x = 3700-3550

=> 2x = 150

=> x = 75%

second isotope is the heaviest one

so it's percentage is (100-x)%

=> (100-75)%

=> 25%

so answer is (3) 25%