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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.