1. Law of Conservation of Mass

2. What do you observe? Coal is completely burned START END
The weight of the jar and its contents is 100 g
The weight of the jar and its contents remains the same. Mass is conserved.

3. Reactants  Products 100g total = 100g total
In a chemical rxn, matter is neither created nor destroyed
Therefore, chemical rxns must be balanced in terms of mass
The amount of mass you start with must equal to the mass of the products (final)
Law: Mass of Reactants =Mass of Products
Reactants  Products
100g total = 100g total

4. The mass on both sides of the arrow must be equal
Ex 1 According to the law of conservation of mass, how much zinc was present in the zinc carbonate? + = +
256g
256g
REMEMBER !!!
The mass on both sides of the arrow must be equal
256g – 152g = 104g of ZINC

5. CuCO3(s)  CuO(s) + CO2(g) 123.6 g 79.6 g ? g 44.0 g
According to the law of conservation of mass, how much carbon dioxide was present in the copper (II) carbonate?
CuCO3(s)  CuO(s) + CO2(g)
123.6 g
79.6 g
? g +
44.0 g
123.6g
123.6g
What are the reactants?
CuCO3
123.6 g
What is the mass of reactants?
Mass?
What are the products?
CuO & CO2
What is the mass of products?
79.6 g + CO2
Answer: – 79.6 = 44.0g of CO2

6. For the Last Two Examples
You can set every Chemical Equation up like an Algebraic Equation and solve it for the variable. For instance this equation,
can be written algebraically using the masses given on the previous slide like below. Now you only have to solve for x!
123.6 [g] = 79.6 [g] x
CuCO3(s)  CuO(s) + CO2(g)

7. The following symbols each represent one atom of different elements:
Which of the sections on the left correctly represent a chemical equation showing the law of conservation of mass in a chemical reaction? I II
III IV

8. The following symbols each represent one atom of different elements:
Which of the sections on the left correctly represent a chemical equation showing the law of conservation of mass in a chemical reaction? I II
III IV