Law of Conservation of Mass Antoine Lavoisier 1775

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. 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.

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

Bases n n n Baking Soda is a Base Chemical name is Sodium Bicarbonate Chemical formula: Na. HCO 3

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

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

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 23 g Hx 1 1 g Cx 1 12 g OX 3 16(3) = 48 g Na. HCO 3 84 g

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

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

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

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

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 23 g 1 g Hx 3 1(3) = 3 g OX 2 16(2) = 32 g Cx 2 12(2) = 24 g 16 g 12 g CH 3 CO 2 Na 82 g

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

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

Mass Reactants = Mass Products Mass of 6 atoms Mass of 8 atoms Na. HCO 3 + CH 3 CO 2 H 84 g + 60 g Reactants 14 atoms = 144 g H 2 O + CH 3 CO 2 Na + CO 2 Water 18 g + Mass of 3 atoms Sodium Acetate 82 g Mass of 8 atoms Carbon Dioxide 44 g = + 144 g 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. n n 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. 5 g Sodium Bicarbonate ? g CO 2 5 g x 44 g = 2. 6 g CO 2 84 g

How can we measure the mass of gas produced? n 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? n 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 n n 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. 6 g 0. 001975 g/cm 3 V = 1, 316 cm 3

Circumference Calculation n What should be the circumference of the balloon, if it holds 1, 316 cm 3 of CO 2? V = C 3 6π2 where C = Circumference V 6π2 = 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? n 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 n Calculated Circumference: 42. 7 cm n Measured Circumference: n Explain Any Difference

Conclusion n 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 n 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.