We are given the following chemical reaction:
so2(g) + 2cl2(g) → socl2(g) + cl2o(g)
We see from the chemical reaction that for every 2 moles of Cl2, there is 1 mole of Cl2O that is produced. Therefore the number of moles of Cl2O is:
moles Cl2O = 0.400 mol Cl2 * (1 mole Cl2O / 2 mole Cl2)
moles Cl2O = 0.200 mol Cl2O