What is the average mass of a single chlorine molecule, cl2?

Okay, so the atomic mass of Cl2 is 35.5, so all you would have to do is multiply that by 2 to get 71.

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).