# Law of Conservation of Mass Law of Definite Proportions

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Law of Conservation of Mass Law of Definite Proportions
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 B C D E Reactants Products Mass A + Mass B = 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. 3

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

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

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

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

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

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

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

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

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

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

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

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 g/cm3 V = m d V = g g/cm3 V = 1,316 cm3

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

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

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

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