Question

Chlorine has two natural isotopes: Cl-37 and Cl-35. Hydrogen reacts with chlorine to form the compound HCl. Would a given amount of hydrogen react with different masses of the two chlorine isotopes? Does this conflict with the law of definite proportions? why or why not?

Answer 1

Answer: Yes, a given amount of Hydrogen would react with different masses of the two isotopes of chlorine, and no, this does not conflict with the Law of Definite proportions

Explanation:

About 76% percent of Cl is found in the Cl-35 isotope, and about  24% in the Cl.37 isotope. that means that about 24% of Cl nuclei have 2 more neutrons than the average Cl nucleus.

So, if $H_2$ reacts with $Cl_2$, 76% of the Hydrogen that reacted will react with Cl-35, and the rest will react with Cl-37. Why does this not conflict with the law of definite proportions? Because each Hydrogen atom ends up paired to a single Chlorine atom! Moreover, the proportion of Cl-35 to Cl-37 remains constant in all samples of Chlorine that are naturally found, thus we will always find the same proportion of Chlorine to Hydrogen in any HCl sample we come across.  Thus the weight of a mol of Cl will always be $35*0.76+37*0.24=35.48$ or 35.45 if we had done this calculation with more significant digits.

Therefore 1 mol of $H_2$ (2 grams) will always react with 1 mol of $Cl_2$ (35.45g*2=70.9 grams), and this is a definite proportion.