Question

Why does the mass of the flask and contents change?

Answer 1

Answer:

Mass of the flask does not change but the content of the flask has changed due to reaction in the flask.

Answer 2

In the reaction flask both the reactants combined to give new products. Yes. We should put a cork on the mouth of the flask so that no other gases in the air combine or get added into the reaction, neither any gas formed in the reaction should escape. No. The mass of the flask before and after reaction do not change.