3.3 - Conservation of Mass
The law of conservation of mass
- In any chemical reaction, the total mass of the products equals the total mass of the reactants.
- This is the law of conservation of mass, and it holds for every reaction.
- No atoms are created or destroyed; they're only rearranged into new substances. Because every starting atom is present in the products, total mass cannot change.
Calculating an unknown mass
- Conservation of mass lets you calculate a missing mass by arithmetic.
- If 12 g of carbon reacts completely with 32 g of oxygen, the mass of carbon dioxide produced must be 12 + 32 = 44 g.
Demonstrating conservation in a closed system
- A closed system is a sealed container that no substances can enter or leave.
- When a precipitation reaction is carried out in a closed flask on a balance, an insoluble solid (precipitate) forms, but the mass reading on the balance does not change:
- CuSO4(aq) + 2NaOH(aq) → Cu(OH)2(s) + Na2SO4(aq)
TermDefinition
Law of conservation of massThe rule that no atoms are created or destroyed in a chemical reaction, so the total mass of the products equals the total mass of the reactants.
Closed systemA system in which no substances can enter or leave, although energy can be transferred.
PrecipitateAn insoluble solid that forms in a solution during a chemical reaction, usually when two solutions are mixed.