Question

Are the weights of the reactants and products same or different.

Answer 1

acoording to law of conservation of mass... it would be same... it may also involve conversion of energy which effects the weight of the product

Answer 2

Their is no loss or gain of atoms, once you balance the equation, so the products weight the same as the reactants. The fact that H2 and Cl2 are gases doesn't affect the weight of the compounds used, as long as the reaction is done in a closed container and you don't lose any.