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Stoichiometry

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Review of Molar Mass The molar mass (MM) of a compound is all the atomic masses for the molecule (or compound) added up. Ex. Molar mass of CaCl 2 Avg. Atomic mass of Calcium = 40.08g Avg. Atomic mass of Chlorine = 35.45g Molar Mass of calcium chloride = 40.08 g/mol Ca + (2 X 35.45) g/mol Cl 110.98 g/mol CaCl 2 20 Ca 40.08a 17 Cl 35.45 Cl

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Practice Calculate the Molar Mass of calcium phosphate Formula = Masses elements: Ca = 40, P = 31, O = 16, Molar Mass = 3x40 + 2x31 + 8x16 = 310 g/mol Ca 3 (PO 4 ) 2

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molar mass Avogadros number Grams Moles particles Everything must go through Moles!!! Calculations

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Chocolate Chip Cookies!! 1 cup butter 1/2 cup white sugar 1 cup packed brown sugar 1 teaspoon vanilla extract 2 eggs 2 1/2 cups all-purpose flour 1 teaspoon baking soda 1 teaspoon salt 2 cups semisweet chocolate chips Makes 3 dozen What is the ratio of eggs to butter to make 3 dozen cookies? What is the ratio of butter to chocolate chips? How many eggs would we need to make 9 dozen cookies? How much brown sugar would I need if I had 1 ½ cups white sugar? 2 eggs 1 cup butter 1 to 2

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Cookies and Chemistry…Huh!?!? Just like chocolate chip cookies have recipes, chemists have recipes as well Instead of calling them recipes, we call them chemical equations Furthermore, instead of using cups and teaspoons, we use moles Lastly, instead of eggs, butter, sugar, etc. we use chemical compounds as ingredients

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Chemistry Recipes Be sure you have a balanced reaction before you start! Example: 2 Na + Cl 2 2 NaCl This reaction tells us that by mixing 2 moles of sodium with 1 mole of chlorine we will get 2 moles of sodium chloride How much Na and Cl 2 would be need if we wanted to make 4 moles of NaCl? 4 moles of Na and 2 moles of Cl 2

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Practice Write the balanced reaction for hydrogen gas reacting with oxygen gas. 2 H 2 + O 2 2 H 2 O What if we had 50 moles of hydrogen, how much oxygen would react with it? How much water would be produced? 50 mol H 2 x 1 mol O 2 = 25 mol O 2 2 mol H 2 50 mol H 2 = 50 mol H 2 O (they have a 1:1 ratio) Mole ratio

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Mole Ratios The mole ratio is used to switch between substances in a chemical equation. Example: How many moles of chlorine are needed to react with 5 moles of sodium? 2 Na + Cl 2 2 NaCl 5 moles Na 1 mol Cl 2 2 mol Na = 2.5 moles Cl 2

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Mole-Mole Conversions How many moles of sodium chloride will be produced if 2.6 moles of chlorine gas reacts with excess sodium metal? 2 Na + Cl 2 2 NaCl 2.6 moles Cl 2 x 2 mol NaCl = 5.20 mol of NaCl 1 mol Cl 2

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Mole-Mass Conversions Most of the time in chemistry, the amounts are given in grams instead of moles Example: How many grams of chlorine are required to react completely with 5.00 moles of sodium to produce sodium chloride? 2 Na + Cl 2 2 NaCl 5.00 moles Na 1 mol Cl 2 70.90g Cl 2 2 mol Na 1 mol Cl 2 = 177g Cl 2 Mole ratio Molar mass

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Practice Calculate the mass in grams of Iodine required to react completely with 0.50 moles of aluminum. 2 Al + 3 I 2 2 AlI 3 Given Quantity convert to moles mole ratio convert to final units =190.35g I 2 0.50 moles AlX 3 moles I 2 2 moles Al X 253.8 g I 2 moles

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Mass-Mole We can also start with mass and convert to moles of product or another reactant Calculate the number of moles of ethane (C 2 H 6 ) needed to produce 10.0 g of water 2 C 2 H 6 + 7 O 2 4 CO 2 + 6 H 2 0 10.0 g H 2 O 1 mol H 2 O 2 mol C 2 H 6 18.0 g H 2 O 6 mol H 2 0 = 0.185 mol C 2 H 6 Given quantity Convert to moles Multiply by the Mole ratio

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Consider : 4NH 3 + 5O 2 6H 2 O + 4NO How many grams of H 2 O are produced if 1.9 mol of NH 3 are combined with excess oxygen? How many grams of O 2 are required to produce 0.3 mol of H 2 O? 6 mol H 2 O 4 mol NH 3 x # g H 2 O= 1.9 mol NH 3 51.4 g H 2 O = 18.02 g H 2 O 1 mol H 2 O x 5 mol O 2 6 mol H 2 O x # g O 2 = 0.3 mol H 2 O 8 g O 2 = 32 g O 2 1 mol O 2 x

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Mass-Mass Conversions Most often we are given a starting mass and want to find out the mass of a product we will get (called theoretical yield) or how much of another reactant we need to completely react with it (no leftover ingredients!) Given Quantity convert to moles mole ratio convert to final units

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Mass-Mass Conversion Ex. How many grams of ammonia are produced when 2.00g of nitrogen react with excess hydrogen. N 2 + 3 H 2 2 NH 3 2.00g N 2 1 mol N 2 2 mol NH 3 17.06g NH 3 28.02g N 2 1 mol N 2 1 mol NH 3 = 2.4 g NH 3

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Practice: Grams to Grams How many grams of calcium nitride are produced when 2.00 g of calcium reacts with an excess of nitrogen? 3 Ca + N 2 Ca 3 N 2 2.00 g Ca x 1 mole Ca x 1 mole Ca 3 N 2 x 148 g Ca 3 N 2 = 2.47 g Ca 3 N 2 40 g 3 mol Ca 1 mole Given amount change to moles multiply by the mole ratio change to (put the one on top that yougrams are solving for)

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Gases and Stoiochiometry Avogadro's Hypothesis - at the same temperature and pressure equal volumes of gas have the same number of particles. At STP 1 mole of any gas has a volume of 22.4 L STP = 0ºC and 1 atmosphere pressure

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19 Gases and Stoichiometry 2 H 2 O 2 (l) ---> 2 H 2 O (g) + O 2 (g) Decompose 1.1 g of H 2 O 2 in a flask with a volume of 2.50 L. What is the volume of O 2 at STP? Bombardier beetle uses decomposition of hydrogen peroxide to defend itself.

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20 Gases and Stoichiometry 2 H 2 O 2 (l) ---> 2 H 2 O (g) + O 2 (g) Decompose 1.1 g of H 2 O 2 in a flask with a volume of 2.50 L. What is the volume of O 2 at STP? Solution 1.1 g H 2 O 2 1 mol H 2 O 2 1 mol O 2 22.4 L O 2 34 g H 2 O 2 2 mol H 2 O 2 1 mol O 2 34 g H 2 O 2 2 mol H 2 O 2 1 mol O 2 = 0.36 L O 2 at STP Molar volume

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21 Gas Stoichiometry: Practice! A. What is the volume at STP of 4.00 g of CH 4 ? 4.00 g CH 4 x 1 mole x 22.4 L = 5.6 L of CH4 16 g 1 mole B. How many grams of CO 2 are produced from the combustion of 20 grams CH 4 at STP? CH 4 + 2 O 2 CO 2 + 2 H 2 O 20 g CH 4 x 1 mole x 1 mole CO 2 x 22.4 L = 28 L CO 2 16 g 1 mole CH 4 1 mole

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