The chlorine gas has a molecular formula Cl2. In table periodic, the molar mass of one atom of chlorine is 35.5 g/mol, so the molar mass of Cl2 is 71 g/mol.
Molar mass, mass and number of moles are related by the equation below:
Where n is the number of moles, m is the mass, and MM is the molar mass of the compound.