Question: how many moles of chlorine molecules are needed?
Answer: Chlorine has an atomic weight of 35.45. A mole of Cl2 molecules would contain two moles of chlorine atoms, each mole weighing 35.45 g. Thus the mass needed to obtain Avogadro's number of Cl2 molecules is equal to 70.90 g. The molar mass of Cl2 is 70.90 g/mol.