STOICHIOMETRY. Engr. Yvonne Ligaya F. Musico 1

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1 STOICHIOMETRY Engr. Yvonne Ligaya F. Musico 1

2 Stoichiometry The study in chemistry dealing with calculations based on balanced chemical equations. The branch of chemistry dealing with mass relationships Engr. Yvonne Ligaya F. Musico 2

3 Interrelationship between Mole,Molar Mass and Number of Particles Engr. Yvonne Ligaya F. Musico 3

4 Flowchart Atoms or Molecules Divide by X Multiply by 6.023X Moles Multiply by atomic/molar mass from periodic table Divide by atomic/molar mass from periodic table Mass (grams) Engr. Yvonne Ligaya F. Musico 4

5 Calculations molar mass Avogadro s number Grams Moles particles Everything must go through Moles!!! Engr. Yvonne Ligaya F. Musico 5

6 Calculations in Stoichiometry Mole to mole Mole to mass Mass to mole Mass to mass Mass to volume Engr. Yvonne Ligaya F. Musico 6

7 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 How many eggs are needed to make 3 dozen cookies? How much butter is needed for the amount of chocolate chips used? 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? Engr. Yvonne Ligaya F. Musico 7

8 Cookies and Chemistry Huh!?!? Just like chocolate chip cookies have recipes, chemists have recipes as well Instead of calling them recipes, we call them reaction equations Furthermore, instead of using cups and teaspoons, we use moles Lastly, instead of eggs, butter, sugar, etc. we use chemical compounds as ingredients Engr. Yvonne Ligaya F. Musico 8

9 Chemistry Recipes Looking at a reaction tells us how much of something you need to react with something else to get a product (like the cookie recipe) 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 chlorine What if we wanted 4 moles of NaCl? 10 moles? 50 moles? Engr. Yvonne Ligaya F. Musico 9

10 Practice For the balanced reaction for hydrogen gas reacting with oxygen gas. 2 H 2 + O 2 2 H 2 O How many moles of reactants are needed? What if we wanted 4 moles of water? What if we had 3 moles of oxygen, how much hydrogen would we need to react and how much water would we get? What if we had 50 moles of hydrogen, how much oxygen would we need and how much water produced? Engr. Yvonne Ligaya F. Musico 10

11 Mole Mole Calculations Engr. Yvonne Ligaya F. Musico 11

12 Mole Ratios These mole ratios can be used to calculate the moles of one chemical from the given amount of a different chemical Engr. Yvonne Ligaya F. Musico 12

13 Example 1 How many moles of chlorine is needed to react with 5 moles of sodium (without any sodium left over)? 2 Na + Cl 2 2 NaCl 5 moles Na 1 mol Cl 2 2 mol Na = 2.5 moles Cl 2 Engr. Yvonne Ligaya F. Musico 13

14 Example 2 20 moles of CO 2 is the average amount exhaled by day. How many moles of LiOH will there have to be to complete this reaction? CO 2 (g) + 2LiOH(s) Li 2 CO 3 (s) + H 2 O(l) 20 moles CO 2 2 mol LiOH 1 mol CO 2 = 40 moles LiOH Engr. Yvonne Ligaya F. Musico 14

15 Practice Exercise How many moles of sodium chloride will be produced if you react 2.6 moles of chlorine gas with an excess (more than you need) of sodium metal? Engr. Yvonne Ligaya F. Musico 15

16 Mole-Mass Calculation Engr. Yvonne Ligaya F. Musico 16

17 Mole-Mass Conversions Most of the time in chemistry, the amounts are given in grams instead of moles We still go through moles and use the mole ratio, but now we also use molar mass to get to grams Engr. Yvonne Ligaya F. Musico 17

18 Mole-Mass Conversions 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 g Cl 2 2 mol Na 1 mol Cl 2 = 177g Cl 2 Engr. Yvonne Ligaya F. Musico 18

19 Practice Exercise Calculate the mass in grams of Iodine required to react completely with 0.50 moles of aluminum. Engr. Yvonne Ligaya F. Musico 19

20 Mass-Mole Calculation Engr. Yvonne Ligaya F. Musico 20

21 Mass-Mole We can also start with mass and convert to moles of product or another reactant We use molar mass and the mole ratio to get to moles of the compound of interest Engr. Yvonne Ligaya F. Musico 21

22 Example 1 Calculate the number of moles of ethane (C 2 H 6 ) needed to produce 10.0 g of water 2 C 2 H O 2 4 CO H g H 2 O 1 mol H 2 O 2 mol C 2 H g H 2 O 6 mol H 2 0 = mol C 2 H 6 Engr. Yvonne Ligaya F. Musico 22

23 Example 2 If 8.00 g of SO 2 reacts, a. How many moles of CS 2 are formed? b. How many moles of CO 2 are formed? 3C (s) + 2SO 2 (g) CS 2 (l) + 2CO 2 (g) Engr. Yvonne Ligaya F. Musico 23

24 Solution (a) 3C (s) + 2SO 2 (g) CS 2 (l) + 2CO 2 (g) 8.00 g SO 2 1 mol SO 2 1 mol CS g SO 2 2 mol SO 2 = 0.06 mol CS 2 Engr. Yvonne Ligaya F. Musico 24

25 Solution (b) 3C (s) + 2SO 2 (g) CS 2 (l) + 2CO 2 (g) 8.00 g SO 2 1 mol SO 2 2 mol CO g SO 2 2 mol SO 2 = mol CO 2 Engr. Yvonne Ligaya F. Musico 25

26 Practice Calculate how many moles of oxygen are required to make 10.0 g of aluminum oxide Engr. Yvonne Ligaya F. Musico 26

27 Mass-Mass Calculation Engr. Yvonne Ligaya F. Musico 27

28 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!) Now we must go from grams to moles, mole ratio, and back to grams of compound we are interested in Engr. Yvonne Ligaya F. Musico 28

29 Example 1 Calculate how many grams of ammonia are produced when you react 2.00g of nitrogen with excess hydrogen. N H 2 2 NH g N 2 1 mol N 2 2 mol NH g NH g N 2 1 mol N 2 1 mol NH 3 = 2.4 g NH 3 Engr. Yvonne Ligaya F. Musico 29

30 Example 2 How many grams of Tin (II) fluoride are produced when 30.0 grams of HF react? Sn (s) + 2HF (g) SnF 2 (s) + H 2 (g) 30.0g HF 1 mol HF 1 mol SnF 2 157g SnF g HF 2mol HF 1 mol SnF 2 = g SnF 2 Engr. Yvonne Ligaya F. Musico 30

31 Practice How many grams of calcium nitride are produced when 2.00 g of calcium reacts with an excess of nitrogen? Engr. Yvonne Ligaya F. Musico 31

32 Mass Volume Calculation Engr. Yvonne Ligaya F. Musico 32

33 Avogadro s Hypothesis It states that one mole of a gas occupies 22.4 liters volume at standard condition At Standard Condition: P = 760 mm Hg = 1 atm T = K Engr. Yvonne Ligaya F. Musico 33

34 Example 1 How many grams of zinc must react with sulfuric acid to produce 1000 ml of hydrogen at STP? Zn (s) + H 2 SO 4 (aq) ZnSO 4 (s) + H 2 (g) 1.0 L H 2 1 mol H 2 1 mol Zn 65.4 g Zn 22.4 L H 2 1 mol H 2 1 mol Zn = 3.0 g Zn Engr. Yvonne Ligaya F. Musico 34

35 Example 2 How many liters of oxygen is needed to react with 10.0 g of carbon to form carbon monoxide at STP? 2C (s) + O 2 (aq) 2CO (g) 10.0 g C 1 mol C 1 mol O L O g C 2 mol C 1 mol O 2 = 9.33 L O 2 Engr. Yvonne Ligaya F. Musico 35

36 Practice 1. What is the volume of oxygen that will react with 30.0 grams of magnesium to form magnesium oxide? 2. How many grams of calcium is needed to react with hydrochloric acid to form calcium chloride and 2.6 liters of hydrogen? Engr. Yvonne Ligaya F. Musico 36

37 Limiting and Excess Reactant Engr. Yvonne Ligaya F. Musico 37

38 Limiting Reactant: The Sandwich Equation 2 pieces of bread + 3 slices meat + 1 slice cheese sandwich Now assume you have the following quantities of ingredients: a. 20 slices of bread b. 24 slices of meat c. 12 slices of cheese How many sandwiches can you make? Engr. Yvonne Ligaya F. Musico 38

39 2 pieces of bread + 3 slices meat + 1 slice cheese sandwich Number of Sandwich can be made: a. For 20 slices of bread = can make 10 sandwiches b. For 24 slices of meat = can make 8 sandwiches c. For 12 slices of cheese = can make 12 sandwiches How many sandwiches can you make? There 8 sandwiches that can be made out of the available ingredients because the number of meat limits the number of product or sandwich. Engr. Yvonne Ligaya F. Musico 39

40 Limiting and Excess Reactant Most of the time in chemistry we have more of one reactant than we need to completely use up other reactant. That reactant is said to be in excess (there is too much). The other reactant limits how much product we get. Once it runs out, the reaction s. This is called the limiting reactant. Engr. Yvonne Ligaya F. Musico 40

41 Limiting and Excess Reactant Limiting Reactant the reactant that controls the amount of product formed in chemical reaction Excess Reactant the substance that is not used up completely in a reaction Engr. Yvonne Ligaya F. Musico 41

42 Limiting Reactant To find the correct answer, we have to try all of the reactants. We have to calculate how much of a product we can get from each of the reactants to determine which reactant is the limiting one. The lower amount of a product is the correct answer. Engr. Yvonne Ligaya F. Musico 42

43 Limiting Reactant The reactant that makes the least amount of product is the limiting reactant. Once you determine the limiting reactant, you should ALWAYS start with it! Be sure to pick a product! You can t compare to see which is greater and which is lower unless the product is the same! Engr. Yvonne Ligaya F. Musico 43

44 Limiting Reactant: Example 10.0g of aluminum reacts with 35.0 grams of chlorine gas to produce aluminum chloride. Which reactant is limiting, which is in excess, and how much product is produced? 2 Al + 3 Cl 2 2 AlCl 3 Start with Al: 10.0 g Al 1 mol Al 2 mol AlCl g AlCl 3 Now Cl : g Al 2 mol Al 1 mol AlCl 3 = 49.4g AlCl g Cl 2 1 mol Cl 2 2 mol AlCl g AlCl g Cl 2 3 mol Cl 2 1 mol AlCl 3 = 43.9g AlCl 3 Engr. Yvonne Ligaya F. Musico 44

45 LR Example Continued We get 49.4g of aluminum chloride from the given amount of aluminum, but only 43.9g of aluminum chloride from the given amount of chlorine. Therefore, chlorine is the limiting reactant. Once the 35.0g of chlorine is used up, the reaction comes to a complete. Engr. Yvonne Ligaya F. Musico 45

46 Limiting Reactant Practice 15.0 g of potassium reacts with 15.0 g of iodine. Calculate which reactant is limiting and how much product is made. Engr. Yvonne Ligaya F. Musico 46

47 Finding the Amount of Excess By calculating the amount of the excess reactant needed to completely react with the limiting reactant, we can subtract that amount from the given amount to find the amount of excess. Can we find the amount of excess potassium in the previous problem? Engr. Yvonne Ligaya F. Musico 47

48 Finding Excess Practice 15.0 g of potassium reacts with 15.0 g of iodine. 2 K + I 2 2 KI We found that Iodine is the limiting reactant, and 19.6 g of potassium iodide are produced g I 2 1 mol I 2 2 mol K 39.1 g K 254 g I 2 1 mol I 2 1 mol K = 4.62 g K USED! 15.0 g K 4.62 g K = g K EXCESS Given amount of excess reactant Amount of excess reactant actually used Note that we started with the limiting reactant! Once you determine the LR, you should only start with it! Engr. Yvonne Ligaya F. Musico 48

49 Limiting Reactant: Recap 1. You can recognize a limiting reactant problem because there is MORE THAN ONE GIVEN AMOUNT. 2. Convert ALL of the reactants to the SAME product (pick any product you choose.) 3. The lowest answer is the correct answer. 4. The reactant that gave you the lowest answer is the LIMITING REACTANT. 5. The other reactant(s) are in EXCESS. 6. To find the amount of excess, subtract the amount used from the given amount. 7. If you have to find more than one product, be sure to start with the limiting reactant. You don t have to determine which is the LR over and over again! Engr. Yvonne Ligaya F. Musico 49

50 Theoretical and Percent Yield Engr. Yvonne Ligaya F. Musico 50

51 Theoretical yield the predicted amount of product Percentage Yield = Actual Yield x100 Theoretical Yield Engr. Yvonne Ligaya F. Musico 51

52 Sample Problem Barium sulfide,bas, which is used as a white pigment in paints, is prepared through the following reaction: BaSO 4 + 4C BaS + 4CO Suppose you have 38 grams of BaSO 4 and 5 grams of carbon. What is the theoretical yield? If the reaction actually produced grams of BaS, what is the percentage yield? Engr. Yvonne Ligaya F. Musico 52

53 Practice Aluminum and chlorine react with each other to form aluminum chloride according to the following equation: 2Al + 3Cl 2 2AlCl 3 Suppose you had 20.0 grams Al and 30.0 grams Cl 2. What would be the theoretical yield? If the actual yield is 31.8 grams AlCl 3, what is the percentage yield? Engr. Yvonne Ligaya F. Musico 53

54 Thanks for Listening Engr. Yvonne Ligaya F. Musico 54