There's only one type of molecule with a distinct mass in a sample of chlorine gas that exists entirely as diatomic molecules.

Here's why:

* Chlorine gas exists as Cl₂: This means each chlorine molecule is composed of two chlorine atoms bonded together.

* Isotopes: Chlorine has two naturally occurring isotopes: chlorine-35 (³⁵Cl) and chlorine-37 (³⁷Cl).

* Possible Combinations: Since each chlorine molecule is Cl₂, there are only a few possible combinations:

* ³⁵Cl-³⁵Cl

* ³⁵Cl-³⁷Cl

* ³⁷Cl-³⁷Cl

While there are three possible combinations, they all have different masses, but only one type of molecule with a distinct mass exists in the sample.