If a gas is produced during a chemical reaction, what precaution should be taken while verifying the law of conservation of mass?

Correct answer: The reaction should be carried out in a closed container, and the total mass should be measured before and after the reaction.

According to the law of conservation of mass, the total mass remains constant during a chemical reaction. In an open container, a gas produced may escape, so its mass will not be included in the measurement. Therefore, the reaction should be performed in a closed container and the total mass should be measured before and after the reaction. Measuring only the remaining solid is incorrect because the gas is also a product of the reaction.