How can you prove that mass is being conserved when adding baking soda to vinegar?

2 answers

Well, every substance has mass. You can try this out but weighing the vinegar before you add baking soda and weigh the vinegar after you add baking soda.
Heart's suggestion, although partly on the mark, won't work because a gas is evolved in the process. Vinegar is acetic acid, CH3COOH. Vinegar + baking soda (NaHCO3) reacts as follows:
CH3COOH + NaHCO3 ==>CO2 + H2O + CH3COONa
To prove the conservation of mass, one would need to enclose this reaction in a sealed vessel and weigh before and after the reaction. Alternatively, one could weigh the reactants, add them, funnel the CO2 gas through an absorbing train to catch the CO2, and weigh the products (CO2 in the absorbant, CH3COONa and water in the reaction vessel).