1. Law of Conservation of Mass Law of Definite Proportions
Antoine Lavoisier, ~ 1775
Law of Definite Proportions
J.L. Proust, 1799

2. Law of Conservation of Mass
In a chemical reaction, the Law of Conservation of Mass states that the Mass of the Reactants must equal the Mass of the Products.
A B
C D E
Reactants
Products
Mass A + Mass B = Mass ( C + D + E )

3. Law of Definite Proportions
Any pure compound only contains the same elements in the same proportion by mass.
H2O
Define proportion: the ratio that relates one part to another part, or relates one part to the whole.
Example: A large proportion of the people present in this classroom are students. 3

4. Acids Vinegar is an Acid Chemical name is Acetic Acid
Chemical formula:
CH3CO2H

5. Bases Baking Soda is a Base Chemical name is Sodium Bicarbonate
Chemical formula:
NaHCO3

6. Acids React with Bases Reactants = Product Acid + Base A Salt Water
Gas (sometimes)
Vinegar + Baking Soda
Sodium Acetate
Water (H2O)
Carbon Dioxide
Mass of Reactants
Mass of Products = = =

7. Hypothesis NaHCO3 + CH3CO2H H2O + CH3CO2Na + CO2
If reactant is 84 grams of baking soda, then by proportion, a product is 44 g of carbon dioxide.
NaHCO3 + CH3CO2H +
84g
60g =
144g
H2O + CH3CO2Na CO2
Water
Sodium Acetate
Carbon Dioxide
18g +
82g +
44g =
144g

8. Law of Definite Proportions Calculating Mass of Molecule A
Atom
Mass (g) Na
Sodium
23 g H
Hydrogen
1 g C
Carbon
12 g O
Oxygen
16 g
Baking Soda
Sodium Bicarbonate
Na x 1
23g
H x 1 1g
C x 1
12g
O X 3
16(3) = 48g
NaHCO3
84g

9. Law of Definite Proportions Calculating Mass of Molecule B
Atom
Mass (g) H
Hydrogen 1g C
Carbon
12g O
Oxygen
16g
Vinegar
Acetic Acid
H x 4 4g
C x 2
24g
O 2 x 16
32g
CH3 CO2H
60g

10. Law of Definite Proportions Calculating Mass of Molecule B
Atom
Mass (g) H
Hydrogen
1 g C
Carbon
12 g O
Oxygen
16 g
Vinegar
Acetic Acid
H x 4
1(4) = 4g
C x 2
12(2) = 24g
O X 2
16(2) = 32g
CH3 CO2H
60g 10

11. Law of Definite Proportions Calculating Mass of Molecule C
Atom
Mass (g) H
Hydrogen O
Oxygen
Water
Dihydrogen Monoxide H O
H2 O 11

12. Law of Definite Proportions Calculating Mass of Molecule C
Atom
Mass (g) H
Hydrogen 1g O
Oxygen
16 g
Water
Dihydrogen Monoxide
H x 2
= 2g
O X 1
16g
H2 O
18g 12

13. Law of Definite Proportions Calculating Mass of Molecule D
Atom
Mass (g) Na
Sodium
23 g H
Hydrogen
1 g O
Oxygen
16 g C
Carbon
12 g
A Salt
Sodium Acetate
Na x 1
23g
H x 3
1(3) = 3g
O X 2
16(2) = 32g
C x 2
12(2) = 24g
CH3 CO2Na
82g

14. Law of Definite Proportions Calculating Mass of Molecule E
Atom
Mass (g) C
Carbon O
Oxygen
Gas
Carbon Dioxide C O
CO2

15. Law of Definite Proportions Calculating Mass of Molecule E
Atom
Mass (g) C
Carbon
12 g O
Oxygen
16 g
Gas
Carbon Dioxide
C x 1
12g
O X 2
16(2) = 32g
CO2
44g 15

16. Mass Reactants = Mass Products
14 atoms
Mass of 6 atoms
Mass of 8 atoms
NaHCO3 + CH3CO2H +
84g
60g =
144g
H2O + CH3CO2Na CO2
Water
Sodium Acetate
Carbon Dioxide =
144g
18g +
82g +
44g
Mass of 3 atoms
Mass of 8 atoms
Mass of 3 atoms
Products
14 atoms

17. Test Hypothesis
To shorten the reaction time, we want to use only a small amount of baking soda.
If reactant is 84 grams of baking soda, then we would get 44 grams of carbon dioxide.
But if we use only 5 grams of baking soda, then by proportion, the product is 2.6 grams of carbon dioxide.
5g Sodium Bicarbonate ? g CO2
5g x 44g = 2.6g CO2
84g

18. How can we measure the mass of gas produced?
Subtract the mass of the bottle + cap after the gas is released from the mass of the bottle + cap before the CO2 is released.
The value should less than 2.6 g
because about 10% of the CO2
remains dissolved in the
water solution.

19. How do we Measure the Volume of a Gas?
If we can measure the circumference of a sphere that traps the gas, such as a balloon, then we can calculate the volume of the gas.

20. Volume Calculation What is the volume of 2.6 grams of CO2?
The density of CO2 is g/cm3
V = m d
V = g
g/cm3
V = 1,316 cm3

21. Circumference Calculation
What should be the circumference of the balloon, if it holds 1,316 cm3 of CO2?
V = C where C = Circumference
6π2
V6π2 = C3
1,316 cm3 x 6 ( x ) = C3
42.7 cm = C

22. How do I Calculate the Mass of a Gas?
If we can measure the volume of the gas and we know its density, then we use D = m/V:
Density (D) = Mass (m)
Volume (V) or
Volume (V) x Density (D) = Mass (m)

23. Comparing Our Measurements with Our Calculations
Calculated Circumference:
42.7 cm
Measured Circumference:
Explain Any Difference

24. My hypothesis……. was supported
Conclusion
My hypothesis……. was supported
by my data because the mass of all the products of this chemical reaction was equal to mass of all the reactants 24

25. Conclusion Continued
I know that this reaction obeys the Law of Conservation of Mass because I used the Law of Definite Proportions to predict the mass of carbon dioxide, and my results matched my prediction within the +/- margin of uncertainty caused by the carbon dioxide that remains dissolved in the water. 25