Chlorine, consisting of two isotopes of mass numbers 35 and 37, has an atomic mass of 35.5. The relative abundance of the isotope of mass number 37 is?

Chlorine, consisting of two isotopes of mass numbers 35 and 37, has an atomic mass of 35.5.

The relative abundance of the isotope of mass number 37 is?

• A. 20
• B. 25
• C. 50
• D. 75
• 
  
• Let the relative abundance of chlorine with mass of 37 be x%,
• :. the relative abundance of isotopes mass 35=(100-x)%.
•  Total mass of isotopes of 37= 37 x,
•  while that of 35=(100-x) 
• :. Total mass of the 100 atoms=37 x + 35(100-x)=35.5  
• 37 x +3500-35x=35.5 
•  :.Mean mass of chlorine atom=(2x+3500)/100=35.5  
• 2 x+3500=3550; x=50/2=25

METHOD TWO

Let A be fraction of Cl-35
And B be fraction of Cl-37
A + B = 1
So B = 1 - A ....eqn1
35 × A + 37 × B = 35.5
35A + 37(1 - A) = 35.5 sub B in eqn1
35A + 37 - 37A = 35.5
-2A = -1.5
A = 0.75

B = 1 - 0.75
B = 0.25

Percentage of Cl-35 = 0.75 ×100 = 75%
Percentage of Cl-37 = 0.25 × 100 = 25%

Jamb 1990 Question 12

•