From the periodic table, molecular mass of chlorine is 35.5 gm/mol.
This represents the average atomic mass of chlorine.
Isotope 37Cl has molar mass 37 grams
Isotope 35Cl has molar mass 35 grams
Now, assume that the percentage of abundance of 37Cl is y. This means that the percentage of abundance of 35Cl is 1-y.
We can calculate y as follows:
37y + 35(1-y) = 35.5
37y + 35 - 35y = 35.5
2y = 0.5
y = 0.25
This means that:
the percentage abundance of 37Cl = 0.25 x 100% = 25%
the percentage abundance of 35Cl = (1-0.25) x 100% = 75%
Based on these calculation:
Isotope 35Cl is more abundant.