The percent abundance of Cl−35 and Cl−37 is equal to 100 . Let abundance of 35Cl=x and abundance of 37Cl=100−x. As we know, Atomic weight of 35Cl=34.9689 Atomic weight of 37Cl=36.9695 Atomic mass of Cl=35.45 Average atomic mass =
34.9689(x)+36.9695(100−x)
100
=35.45 x=76% Abundance of Cl−35=76% Abundance of Cl−37=24% Hence, the ratio of their natural abundance is approximately equal to 3:1.