Physical Science / Unit 5: Chemical Reactions
5.2 Conservation of Mass
Notes
The law of conservation of mass: in a chemical reaction, matter is neither created nor destroyed. The total mass of the reactants equals the total mass of the products.
- Atoms are rearranged, not lost, so the number of each kind of atom is the same on both sides.
- Example: 12 g of carbon burns with 32 g of oxygen. The product, carbon dioxide, has a mass of $12 + 32 = 44$ g.
- If a gas escapes from an open container, the measured mass seems to drop, but the total mass is the same.
Practice
4 g of hydrogen reacts completely with 32 g of oxygen. What is the mass of water formed?
$4 + 32 = 36$ g.
A student burns a candle and the candle gets lighter. Why does this not break the law of conservation of mass?
The products CO2 and water vapor have the mass lost by the candle plus the oxygen used.
A reaction produces 50 g of products. If 20 g of one reactant was used, how much of the other reactant was used?
Total reactant mass = total product mass, so $50 - 20 = 30$ g.
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