Question

Chlorine has two stable isotopes, 35Cl and 37Cl. If a sample of chlorine has an average molar mass of 36.19 g/mol, what percent of the sample is composed of the lighter isotope, 35Cl? The exact masses of the isotopes can be found at http://en.wikipedia.org/wiki/Isotopes_of_chlorine

Answer 1

Let fraction of Cl in Cl-35 = x. Fraction of Cl as Cl-37 = 1 - x

we have: mass of Cl = fraction of Cl-35 * 35 + fraction of Cl-37 * 37

36.19 = x * 35 + (1-x) * 37

2x = 37 - 36.19

x = 0.41

% of Cl-35 = fraction * 100 = 0.41 * 100 = 41%