In the reaction #2Na+Cl_2 -> 2NaCl#, how many moles of #Cl_2# react with 100 moles of sodium to form sodium chloride?
Look at the stoichiometry of the reaction. One mole of chlorine gas reacts with 2 mol sodium metal to give 2 mol sodium chloride.
Remember that the mole is a NUMERICAL (or extensive) quantity. I could say just as correctly that 2 dozen sodium atoms react with 1 dozen chlorine molecules to form 2 dozen sodium chloride entities.
You have started with 100 mol sodium metal. Clearly I need 50 mol of chlorine molecules for a stoichiometric reaction. Given these quantities, how many moles of sodium chloride will result?