Question

What supports the idea that mass is conserved in a reaction that produces a gas as a product?

Answer 1

Law of conservation of mass supports the idea

Answer 2

This law expresses that, notwithstanding compound responses or physical changes, mass is moderated — that is, it can't be made or demolished — inside a disengaged framework.

At the end of the day, in a concoction response, the mass of the items will dependably be equivalent to the mass of the reactants.

Beyond any doubt this will make a viable effect.