State the law of conservation of mass. then apply the law to this question: what would be the total mass of the products of a reaction in which 10 grams of water decomposes into elements hydrogen and oxygen?

The law of conservation of mass states that:
"For any closed system, mass is preserved. This means that the total amount of mass remains constant as mass can neither be created nor destroyed".

Applying this to the decomposition we have,
since the total mass of reactants (water) is 10 grams, therefore, the total mass of products (hydrogen and oxygen) is also 10 grams