Answer
Yes, H will react with different masses of Cl.
No, this does not conflict with the law of definite proportion.
Work Step by Step
Hydrogen with react with the mass ratio of 1 gram of Hydrogen to 37 grams of Chlorine with $^{37}Cl$ and 1 gram of Hydrogen to 35 grams of Chlorine with $^{35}Cl$.
This does not contradict the Law of Definite Proportion as these ratios hold whenever Hydrogen reacts with $^{37}Cl$ and $^{35}Cl$, respectively.
