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**Law of Conservation of Mass Law of Definite Proportions**

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

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**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 )

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**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

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**Acids Vinegar is an Acid Chemical name is Acetic Acid**

Chemical formula: CH3CO2H

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**Bases Baking Soda is a Base Chemical name is Sodium Bicarbonate**

Chemical formula: NaHCO3

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**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 = = =

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**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

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**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

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**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

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**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

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**Law of Definite Proportions Calculating Mass of Molecule C**

Atom Mass (g) H Hydrogen O Oxygen Water Dihydrogen Monoxide H O H2 O 11

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**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

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**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

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**Law of Definite Proportions Calculating Mass of Molecule E**

Atom Mass (g) C Carbon O Oxygen Gas Carbon Dioxide C O CO2

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**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

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**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

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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

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**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.

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**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.

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**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

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**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

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**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)

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**Comparing Our Measurements with Our Calculations**

Calculated Circumference: 42.7 cm Measured Circumference: Explain Any Difference

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**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

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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

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