A reaction occurs between two liquid substances in a sealed jar. What will happen to the mass of the sealed jar and is contents after the reaction occurs?
Well, nothing should happen... to the mass, anyway. The identities of what's inside should change as a result of the reaction, but mass is always conserved in a closed system.
The law of conservation of mass applies in a closed system, wherein the mass at an initial state will be equal to the mass at any given final state, since no mass has (presumably) escaped.
(It's possible that some heat can escape if the jar is not properly insulated, but that is not mass escaping.)