The first thing you have to do is convert the 500 g of Cl2 into moles. So you get the mass of Cl which is 35.5, but Cl is diatomic by itself so you multiply 35.5 x 2 and get 71. now on the third column you are comparing the mol ratios of the equation. The equation is 2Na + Cl2 ---> 2NaCl, so you see that there is no coefficient in front of Cl2 so that means there is 1 mol, and then you see that in the product there is a coefficient of 2 in front of NaCl so that means there is 2 moles. Then finally you have to convert the moles backs into grams where you just add the masses of Na and Cl . You multiply across the top and then divide that number by the bottom row.
