Antoine Lavoisier, ~ 1775
J.L. Proust, 1799

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 )

Any pure compound only contains the same elements in the same proportion by mass.
H
2
O
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.

 Vinegar is an Acid
 Chemical name is Acetic Acid
 Chemical formula:
CH
3
CO
2
H

 Baking Soda is a Base
 Chemical name is Sodium Bicarbonate
 Chemical formula:
NaHCO
3

Reactants = Product
Acid + Base
Vinegar + Baking Soda
Mass of Reactants
=
=
=
A Salt
Water
Gas (sometimes)
Sodium Acetate
Water (H
2
O)
Carbon Dioxide
Mass of Products

 If reactant is 84 grams of baking soda, then by proportion, a product is 44 g of carbon dioxide.
NaHCO
3
84g
+ CH
3
CO
2
H
+
60g
=
144g
H
2
O + CH
3
CO
2
Na + CO
2
Water
18g
+
Sodium Acetate
82g
+
Carbon Dioxide
44g
=
144g

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

Law of Definite Proportions
Calculating Mass of Molecule B
Atom Mass (g) Vinegar
Acetic Acid
H
Hydrogen
C
Carbon
O
Oxygen
1g
12g
16g
H x 4
C x 2
O 2 x 16
CH
3
CO
2
H
4g
24g
32g
60g

Law of Definite Proportions
Calculating Mass of Molecule B
Atom Mass (g) Vinegar
Acetic Acid
H
Hydrogen
C
Carbon
O
Oxygen
1 g
12 g
16 g
H x 4 1(4) = 4g
C x 2 12(2) = 24g
O X 2 16(2) = 32g
CH
3
CO
2
H 60g

Law of Definite Proportions
Calculating Mass of Molecule C
Atom Mass (g)
H
Hydrogen
O
Oxygen
Water
Dihydrogen Monoxide
H
O
H
2
O

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

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

Law of Definite Proportions
Calculating Mass of Molecule E
Atom Mass (g)
C
Carbon
O
Oxygen
Gas
Carbon Dioxide
C
O
CO
2

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

Mass Reactants = Mass Products
Mass of 6 atoms
NaHCO
3
84g
Mass of 8 atoms
+ CH
3
CO
2
H
+
60g
=
Reactants
14 atoms
144g
H
2
O + CH
3
CO
2
Na + CO
2
Water
18g
+
Sodium Acetate
82g
+
Carbon Dioxide
44g
=
Mass of 3 atoms
144g
Mass of 8 atoms Mass of 3 atoms
Products
14 atoms

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 CO
2
5g x 44g = 2.6g CO
2
84g

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 CO
2 is released.
The value should less than 2.6 g because about 10% of the CO
2 remains dissolved in the water solution.

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.



What is the volume of 2.6 grams of CO
2
?
The density of CO
2 is 0.001975 g/cm 3
V = m d
V = 2.6g
0.001975g/cm 3
V = 1,316 cm 3

 What should be the circumference of the balloon, if it holds 1,316 cm 3 of CO
2
?
V = C 3
6π 2 where C = Circumference
V6π 2 = C 3
1,316 cm 3 x 6 (3.1415 x 3.1415) = C 3
42.7 cm = C

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)

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

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

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.