Final answer:
John and Gina's finding that the mass of reactants and products remained constant at 40 g during a chemical reaction exemplifies the law of conservation of mass, which states that mass is conserved in a chemical reaction.
Step-by-step explanation:
When John and Gina observed that the mass of the reactants before the reaction was 40 g and the mass of the products after the reaction was also 40 g, this directly illustrates the law of conservation of mass. According to this fundamental law, mass is neither created nor destroyed in a chemical reaction. So, the total mass you start with will remain the same after the reaction has occurred.
In the context of the reactions between sodium chloride and silver nitrate, we see this principle at work. For example, if we start with 58.5 grams of sodium chloride and 169.9 grams of silver nitrate (totaling 228.4 grams), after the reaction, we end up with 143.4 grams of silver chloride and 85.0 grams of sodium nitrate, which sum up to the same 228.4 grams.
This constancy of mass is because in a chemical reaction, atoms are rearranged to form new compounds - they do not vanish nor appear out of nowhere. In the reaction between silver nitrate and sodium chloride to form silver chloride and sodium nitrate, the silver chloride precipitates out and can be recovered by filtration, after which we can evaporate the water to recover the sodium nitrate. This demonstrates the molar stoichiometry and the law of conservation of mass in a double replacement reaction.