This problem requires using Avogadro's rule:
The first step is to calculate the number of chlorine molecules in 6.5 moles of chlorine. We use
Avogadro constant to solve this: 1mole = 6.02 x10^23 molecules
So for 6.5 moles:
6.5 x ( 6.02x 10^23)= 3.913 x10^24