A reaction that gives off a gas is followed by weighing the flask over time. Why must the flask stay open?

1. This method reads the rate from the drop in mass of the reaction vessel. 2. Mass only drops if something leaves the vessel, and here that something is the gas. 3. A sealed vessel keeps the gas inside, so the balance reading would never change. 4. The flask therefore has to stay open for the method to work at all. 5. Boiling the acid is not needed and would only add error, so that option is wrong. _Source: Siyavula Physical Sciences Grade 12 (Everything Science, CC BY 4.0), Rate and Extent of Reaction_