chemical reactions do not have weight i think.. they involve electrical bonds breaking and forming , and the end result ( unless something boils off) is the same mass as wat you started off with , just the energy level has changed and the chemicals are bonded to different chemicals .
You might as well ask if electricity or light has weight... they are supposedly waves of energy so they don't ....
Then again i do remember one of my many physics teachers mentioning nuclear fuel loosing like .00001% every time it goes through the reactor , the matter is turned into pure enegry , e=mc 2 ... so possibly in a chemical reaction the reactants loose a portion of its mass to energy , but its so small that it cant' be measured...
Also if it so.. it means that after a few zillion years the universe will be made of pure energy O_O ... That is unless energy turns back into matter .
true crazy talk :p