Chlorine consisting of two isotopes of mass number 35 and 37 in the ratio 3:1 has an atomic mass of 35.5. Calculate the relative abundance of the isotopes of mass number 37?

A. 20
B. 25
C. 60
D. 75

Correct Answer:

Option B – 25

Explanation

Let the relative abundance of oxygen with mass 37 be x%
∴ The relative abundance of the isotope of mass 35 = (100 – x)%. Total mass of isotopes of mass 37 = 37x, while that of 35 = 35(100 – x)
∴ Total mass of the 100 atoms
= 37x + 35(100 – x) = 35.5
= 37x + 3500 – 35x = 35.5
∴ Mean mass of chlorine atom = (2x + 3500)/100 = 35.5
2x + 3500 = 3550; x = 50/2 = 25