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Law of Conservation of Mass
Antoine Lavoisier, ~ 1775
Law of Definite Proportions
J.L. Proust, 1799
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
C
+ B
+
E
Products
Reactants
Mass A + Mass B
+ D
=
Mass ( C + D + E )
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.
Acids



Vinegar is an Acid
Chemical name is Acetic Acid
Chemical formula:
CH3CO2H
Bases



Baking Soda is a Base
Chemical name is Sodium Bicarbonate
Chemical formula:
NaHCO3
Acids React with Bases
Reactants
=
Acid + Base
Vinegar + Baking Soda
Mass of Reactants
Product
A Salt
Water
=
Gas (sometimes)
Sodium Acetate
=
Water (H2O)
Carbon Dioxide
= Mass of Products
Hypothesis

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
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
1g
12 g
16 g
Na x 1
23g
Hx1
1g
Cx1
12g
OX3
16(3) = 48g
NaHCO3
84g
Law of Definite Proportions
Calculating Mass of Molecule B
Atom
Mass (g)
Vinegar
Acetic Acid
H
Hydrogen
C
Carbon
O
Oxygen
1g
Hx4
4g
12g
Cx2
24g
O 2 x 16
32g
CH3 CO2H
60g
16g
Law of Definite Proportions
Calculating Mass of Molecule B
Atom
Mass (g)
Vinegar
Acetic Acid
H
Hydrogen
C
Carbon
O
Oxygen
1g
Hx4
1(4) = 4g
12 g
Cx2
12(2) = 24g
OX2
16(2) = 32g
16 g
CH3 CO2H
60g
Law of Definite Proportions
Calculating Mass of Molecule C
Atom
Mass (g)
Water
H
Hydrogen
O
Oxygen
Dihydrogen Monoxide
H
O
H2 O
Law of Definite Proportions
Calculating Mass of Molecule C
Atom
Mass (g)
Water
H
Hydrogen
O
Oxygen
Dihydrogen Monoxide
1g
Hx2
= 2g
16 g
OX1
16g
H2 O
18g
Law of Definite Proportions
Calculating Mass of Molecule D
Atom
Na
Sodium
H
Hydrogen
O
Oxygen
C
Carbon
Mass (g)
A Salt
Sodium Acetate
23 g
Na x 1
23g
1g
Hx3
1(3) = 3g
OX2
16(2) = 32g
Cx2
12(2) = 24g
16 g
12 g
CH3 CO2Na
82g
Law of Definite Proportions
Calculating Mass of Molecule E
Atom
C
Carbon
O
Oxygen
Mass (g)
Gas
Carbon Dioxide
C
O
CO2
Law of Definite Proportions
Calculating Mass of Molecule E
Atom
C
Carbon
O
Oxygen
Mass (g)
12 g
16 g
Gas
Carbon Dioxide
Cx1
12g
OX2
16(2) = 32g
CO2
44g
Mass Reactants = Mass Products
Mass of 6 atoms
Mass of 8 atoms
Reactants
14 atoms
NaHCO3 + CH3CO2H
84g
+
60g
=
144g
H2O + CH3CO2Na + CO2
Water
18g
+
Mass of 3 atoms
Sodium Acetate
82g
Mass of 8 atoms
Carbon Dioxide
44g =
+
144g
Mass of 3 atoms
Products
14 atoms
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
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.
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.
Volume Calculation


What is the volume of 2.6 grams of CO2?
The density of CO2 is 0.001975 g/cm3
V=m
d
V = 2.6g
0.001975g/cm3
V = 1,316 cm3
Circumference Calculation

What should be the circumference of the balloon,
if it holds 1,316 cm3 of CO2?
V = C3
6π2
where C = Circumference
V6π2 = C3
1,316 cm3 x 6 (3.1415 x 3.1415) = C3
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.