Stoichiometry? help? mole-m?

Your NaCl and Cl2 are in a 2:1 mole ratio (shown by the coefficients) so you can ume that for everyone 2 moles of NaCl reacting, 1 mole of Cl2 is produced. So, if you have 2.56 moles of NaCl is reacted, then 1.28 moles is produced.