Why would a chemical compound with a gas as a product still have the same weight as its reactants?

We did a lab in chemistry, now we have to answer the question why the 3 reactants we had weighed the same when we started mixing them together, the final compound was Na2CO3, CaCl2, and H2SO4 mixed together, this created a gas as one of the products, and there was no lid over the compound, allowing the gas to escape. Why would this still weigh the same as Na2Co3+CaCl2 in one container and H2SO4 in another container, but weighed at the same time.