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: B
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
