Explain Conservation of Mass Practice Test

A student observes that when steel wool rusts, its mass increases. In an experiment, dry steel wool has a mass of 30 g. After sitting in moist air for several days, the rusted steel wool has a mass of 38 g. Which explanation best fits the law of conservation of mass?

A student mixes two solutions in a sealed jar. Reaction: $$\mathrm{AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)}$$. Masses of reactant solutions added:

• $$\mathrm{AgNO_3}$$ solution: 40 g
• $$\mathrm{NaCl}$$ solution: 40 g The jar is sealed before mixing and remains sealed. After the precipitate forms, which total product mass is expected (not including the jar)?