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Chapter 9: Stoichiometry Ridgewood High School scasatelli@ridgewood.k12.nj.us
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Meaning of the word stoikheion-element metry-measure to measure elements method for determining mass and other quantities based on a chemical equation
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Chapter 9.1 Calculating Quantities in Reactions Recipe For 12 Muffins 2 cups all-purpose flour 3 teaspoons baking powder 1/2 teaspoon salt 3/4 cup white sugar 1 egg 1 cup milk 1/4 cup vegetable oil What does this recipe show you?
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Calculating Quantities in Reactions Recipe For 24 Muffins 4 cups all-purpose flour 2 tablespoons baking powder 1 teaspoon salt 1-1/2 cups white sugar 2 egg 2 cups milk 1/2 cup vegetable oil Questions 1. What is the ratio of flour to baking powder? 2. What is the ratio of eggs to milk? 3. How much flour is used in 1 muffin? 4. How much milk is used in 1 muffin?
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Questions 1. What is the ratio of flour to baking powder? 4 cups flour = 2:1 ratio 2 tablespoons of baking powder 2. What is the ratio of eggs to milk? 2 eggs = 1:1 ratio 2 cups milk 3. How much flour is used in 1 muffin? 1 muffin x 4 cups flour =.16 cup of flour 24 muffin
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How does this work? Why can I cross out Muffins in Question 3? Because when you multiple fractions the units that are diagonal from each other cancel out. How many moles are in 50g of H 2 O? (This is a grams to moles conversion problem) 50g H 2 O x 1 mole H 2 O = 2.78 mole H 2 O 18g H 2 O How did you get 18g of H 2 O? Why did you start with 50g of H 2 O?
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Old Conversion Questions How many atoms are in 25 moles of Magnesium? 25 moles Mg x 6.02 x 10 23 atoms Mg 1 mole Mg =1.505 x 10 25 atoms of Mg
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Old Conversion Questions How many atoms are in 3 moles of Magnesium Oxide? 3 moles MgO x 6.02 x 10 23 mlcls MgO 1 mole MgO =1.81 x 10 24 mlcls MgO x 2 atoms 1mlcl MgO =3.61 x 10 24 atoms of Mg and O Atoms and molecules are different, but have the same conversion factor. You need to determine what you are solving for and from.
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Old Conversion Question How many grams are in 1.25 x 10 25 atoms of Sulfur? (2 step) 1.25 x 10 25 atoms S x 1 mole S 6.02 x 10 23 atoms S =20.76 mole S x 32g S =664.45g S 1 mole S
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Recipe vs. Balanced Chemical Equation A balanced chemical equation is like a recipe. 2H 2(g) + O 2(g) 2H 2 O (l) What is the ratio of hydrogen gas to oxygen gas? 2 moles H 2 = 2: 1 ratio 1 mole O 2 What are all the mole ratios in this chemical equation?
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Mole Ratios 2H 2(g) + O 2(g) 2H 2 O (l) What are all the mole ratios in this chemical equation? 2 mol H 2 2 mol H 2 1 mol O 2 1 mol O 2 2 mol H 2 O 2 mol H 2 1 mol O 2 2 mol H 2 O 2 mol H 2 O 2 mol H 2 O 2 mol H 2 1 mol O 2
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Question 2 H 2(g) O 2(g) 2H 2 O (l) How many moles of water will be produced if I have 10 moles of oxygen? Rule #1: start with the number you are given…10 moles O 2 and multiple by the correct mole ratio. 10 moles O 2 x what you are solving for on top ? moles O 2 on bottom to cross off units 10 moles O 2 x ? moles H 2 O ? Moles O 2 Where do you find the # of moles???
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Question 2 H 2(g) O 2(g) 2H 2 O (l) How many moles of water will be produced if I have 10 moles of oxygen? 10 mol O 2 x 2 mol H 2 O = 20 mol H 2 O 1 mol O 2 Where do the mole numbers come from?
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Mole Ratio Questions 2C 8 H 18(g) + 25O 2(g) 16CO 2(g) + 18H 2 O (l) What is the mole ratio of carbon dioxide to oxygen? 16 mole CO 2 25 mol O 2 How many moles of carbon dioxide are produced from 12.5 moles of oxygen gas? 12.5 mol O 2 x 16 mol Co 2 = 8 mol CO 2 25 mol O 2 Multiple 12.5 x 16 and then divide that answer by 25 12.5 x 16 = 200 then 200/25 = 8
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Practice Problems Calculate the amounts requested if 3.30mol Fe 2 O 3 completely reacts according to the following equation. Fe 2 O 3 + 2Al 2Fe + Al 2 O 3 How many moles of aluminum is needed to react with 3.30 mol Fe 2 O 3 ? How many moles of iron is formed from 3.30 mol Fe 2 O 3 ? How many moles of Al 2 O 3 is produced from 3.30 mol Fe 2 O 3 ?
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Answer to Practice Problem Fe 2 O 3 + 2Al 2Fe + Al 2 O 3 3.30mol Fe 2 O 3 x 2mol Al =6.60mol Al 1 mol Fe 2 O 3 3.30mol Fe 2 O 3 x 2mol Fe = 6.60mol Fe 1 mol Fe 2 O 3 3.30mol Fe 2 O 3 x 1mol Al 2 O 3 =3.30mol Fe 2 O 3 1 mol Fe 2 O 3
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