Chapter 9: Stoichiometry Ridgewood High School

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1 Chapter 9: Stoichiometry Ridgewood High School scasatelli@ridgewood.k12.nj.us

2 Meaning of the word  stoikheion-element  metry-measure  to measure elements  method for determining mass and other quantities based on a chemical equation

3 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?

4 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?

5 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

6 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?

7 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

8 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.

9 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

10 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?

11 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

12 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???

13 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?

14 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

15 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 ?

16 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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