1. Stoichiometry (Chapter 12)
Dr. Walker

2. Stoichiometry (stoy-KEY-ah-muh-tree)
Definition: predicting the amounts of reactants and/or products that will be involved in a reaction

3. Chocolate Chip Cookie Recipe
Similar to using recipes to cook
When cooking, recipes require specific amounts of each ingredient to make a set amount of food.
Example:
Chocolate Chip Cookies
2 ½ cups flour ½ cup sugar
¾ teaspoon salt 2 eggs
½ cup butter 1 teaspoon vanilla extract
1 cup brown sugar 1 pkg chocolate chips
1 cup shortening 1 cup chopped nuts
Makes 6 dozen cookies

4. Recipes = Chemical Formulas
When cooking, recipes require specific amounts of each ingredient to make a set amount of food.
Chemical reactions are the exact same thing!!!
We need to know how much material can be made from existing amounts (i.e., how much food is in the pantry) OR how much we need to make a specific amount (how much of everything do I need to make 10 dozen cookies)

5. Chemical Reactions
Chemical change involves a reorganization of the atoms in one or more substances.
C2H5OH + 3O2 ® 2CO H2O
reactants
products
When the equation is balanced it has quantitative significance:
1 mole of ethanol reacts with 3 moles of oxygen to produce 2 moles of carbon dioxide and 3 moles of water – Pay attention to the coefficients!!!!!

6. Chemical Reactions
Chemical change involves a reorganization of the atoms in one or more substances.
C2H5OH + 3O2 ® 2CO H2O
reactants
products
When the equation is balanced it has quantitative significance:
How many moles of ethanol could react with 9 moles of oxygen? If this happened, what products would we get?

7. Chemical Reactions C2H5OH + 3O2 ® 2CO2 + 3H2O reactants products
How many moles of oxygen could react with 9 moles of oxygen? If this happened, what products would we get?
If 1 mole of ethanol reacts with 3 moles of oxygen, then it would require 3 moles of ethanol to react with 9 moles of oxygen.
If 1 mole of ethanol gives 2 moles CO2 and 3 moles H2O, 3 moles of ethanol would yield 3 x 2 = 6 moles CO2 and 3 x 3 = 9 moles H2O

8. Mole Ratios Using the coefficients we can write mole ratios.
Definition: A mole ratio gives the relative amounts of reactants and products using a fraction
The coefficients you used in the recipes are basically mole ratios. You’re doing the same thing.

9. Using Mole Ratios Example: N2 + 3 H2  2 NH3
Give the mole ratio of N2 to NH3

10. Using Mole Ratios Example: 1 N2 + 3 H2  2 NH3
Give the mole ratio of N2 to NH3
Use the coefficients
1 N2 to 2 NH3

11. Using Mole Ratios Example: N2 + 3 H2  2 NH3
Give the mole ratio of H 2 to NH3

12. Using Mole Ratios Example: N2 + 3 H2  2 NH3
Give the mole ratio of H 2 to NH3
Use the coefficients
3 H2 to 2 NH3

13. Mole-Mole Conversions
Write all
Mole-Mole Conversions
Example: N2 + 3 H2  2 NH3
How many moles of ammonia could be produced from 6 moles of N2 and excess H2?

14. Mole-Mole Conversions
Example: N2 + 3 H2  2 NH3
How many moles of ammonia could be produced from 6 moles of N2 and excess H2?
What are we starting with?
6 moles of N2
Whenever you have excess, you have more than enough – you won’t use it in the problem!
6 moles
????

15. Mole-Mole Conversions
Example: N2 + 3 H2  2 NH3
How many moles of ammonia could be produced from 6 moles of N2 and excess H2?
Use the coefficients in a proportion
6 moles
????
1 mole N2
2 moles NH3

16. Mole-Mole Conversions
Example: N2 + 3 H2  2 NH3
How many moles of ammonia could be produced from 6 moles of N2 and excess H2?
6 moles
????
??? = 12 moles NH3
1 mole N2
2 moles NH3

17. Another Mole-Mole Example
2 C8H O CO H2O
How many moles of water can be produced from 1.2 moles of octane and excess O2?

18. Another Mole-Mole Example
2 C8H O CO H2O
How many moles of water can be produced from 1.2 moles of octane and excess O2?
1.2 moles C8H18
?????
2 moles C8H18
18 moles H2O

19. Another Mole-Mole Example
2 C8H O CO H2O
How many moles of water can be produced from 1.2 moles of octane and excess O2?
1.2 moles C8H18
?????
2 moles C8H18
18 moles H2O
???? = 10.8 moles H2O

20. You try… 2 Al + 3 H2SO4 Al2(SO4)3 + 3 H2
How many moles of hydrogen can be produced from 0.24 moles of aluminum?
2 Al2O Al + 3 O2
How many moles of aluminum can be made from 0.35 moles of Al2O3

21. You try… 2 Al + 3 H2SO4 Al2(SO4)3 + 3 H2
How many moles of hydrogen can be produced from 0.24 moles of aluminum?
2 Al2O Al + 3 O2
How many moles of aluminum can be made from 0.35 moles of Al2O3
0.24 moles Al
?????
2 moles Al
3 moles H2
???? = 0.36 moles H2
0.35 moles Al2O3
?????
2 moles Al2O3
4 moles Al
???? = 0.70 moles Al

22. The Next Step… Mass-Mass Stoichiometry
When we measure materials for chemical reactions, the scales measure in grams, not moles
Why can’t we just multiply the original mass times the mole ratio?

23. The Next Step… Mass-Mass Stoichiometry
When we measure materials for chemical reactions, the scales measure in grams, not moles
Why can’t we just multiply the original mass times the mole ratio?
We must account for the difference in molar masses between the two materials we are dealing with

24. Proportion Method “Plug and chug” Plug in the known numbers
Solve for the unknown

25. Mass-Mass Example C3H8 + 5 O2 3 CO2 + 4 H2O
How many grams of H2O could be produced from the combustion of 100 g of propane?

26. Mass-Mass Example C3H8 + 5 O2 3 CO2 + 4 H2O
How many grams of H2O could be produced from the combustion of 100 g of propane?
100 g
????
Mass (start) = 100 g
Coefficient (start) = 1
Molar Mass (start) = g/mole
Mass (finish) = unknown
Coefficient (finish) = 4
Molar Mass (start) = g/mole
Remember – cross multiply, then divide for solving proportions
(100 g x 4 moles x g/mole) / (1 mole x g/mole) = ???

27. Mass-Mass Example C3H8 + 5 O2 3 CO2 + 4 H2O
How many grams of H2O could be produced from the combustion of 100 g of propane?
100 g
????
Grams H2O
100 g C3H8
1 mole C3H8 x g/mole C3H8
4 mole H2O x g/mole H2O
Mass (start) = 100 g
Coefficient (start) = 1
Molar Mass (start) = g/mole
Mass (finish) = g (answer!!)
Coefficient (finish) = 4
Molar Mass (start) = g/mole

28. Some helpful hints… Only use the materials provided in the problem
The compounds not listed are irrelevant to the problem, even if they’re in the reaction
The coefficients and the molar masses are separate
Do not count the coefficient in your molar mass!!!
If you see the word “excess”, cross out that chemical – you will not use it!!!
The word excess just means you’ve got plenty of material to react with the amounts that you have.

29. Another Example 6 Na + Fe2O3 3 Na2O + 2 Fe
How many grams of Fe can be produced by the reaction of 10 g Na and excess iron oxide?

30. Another Example 6 Na + Fe2O3 3 Na2O + 2 Fe
How many grams of Fe can be produced by the reaction of 10 g Na and excess iron oxide?
10 g
????

31. Another Example 6 Na + Fe2O3 3 Na2O + 2 Fe
How many grams of Fe can be produced by the reaction of 10 g Na and excess iron oxide?
Grams Fe
10 g Na
2 mole Fe x g/mole Fe
6 mole Na x g/mole Na

32. Another Example 6 Na + Fe2O3 3 Na2O + 2 Fe
How many grams of Fe can be produced by the reaction of 10 g Na and excess iron oxide?
Grams Fe
10 g Na
2 mole Fe x g/mole Fe
6 mole Na x g/mole Na
Grams Fe = 8.10 g (answer!!)

33. Volume-Volume Stoichiometry
It is difficult to measure gases with a mass, so they are usually measured with a volume
Instead of using the molar mass, use 22.4 L
1 mole of any gas = 22.4 L volume (at 0oC and 1 atm pressure, known as STP)

34. Example
N2 + 3 H NH3
How many liters of ammonia can be made from 15 liters of hydrogen and excess nitrogen?

35. Example
N2 + 3 H NH3
How many liters of ammonia can be made from 15 liters of hydrogen and excess nitrogen?
Liters NH3
15 L
2 mole NH3 x 22.4 L/mole NH3
3 mole H2 x 22.4 L/mole H2
Answer = 10 liters NH3

36. Mixed Stoichiometry
Sometimes, we need to start with one unit and finish with another.
If you’re converting between mass and moles, use the molar mass
If you’re converting between volume and moles, use 1 mole = 22.4 L
If the conversion involves molecules, use Avogadro’s number!

37. Mixed Example 2 KClO3 (s) 2 KCl (s) + 3 O2 (g)
How many liters of oxygen are produced from the decomposition of 244 g KClO3?

38. Mixed Example 2 KClO3 (s) 2 KCl (s) + 3 O2 (g)
How many liters of oxygen are produced from the decomposition of 244 g KClO3?
Remember, since we’re dealing with liters of oxygen, we’ll use 22.4 L instead of the molar mass.

39. Mixed Example 2 KClO3 (s) 2 KCl (s) + 3 O2 (g)
How many liters of oxygen at STP are produced from the decomposition of 244 g KClO3?
244 g KClO3
Liters O2
3 mole O2 x 22.4 L O2
2 mole KClO3 x g/mole KClO3

40. Mixed Example 2 KClO3 (s) 2 KCl (s) + 3 O2 (g)
How many liters of oxygen at STP are produced from the decomposition of 244 g KClO3?
244 g KClO3
Liters O2
3 mole O2 x 22.4 L O2
2 mole KClO3 x g/mole KClO3
Liters O2 = L (answer!!)

41. Another Mixed Example 2 Ca (s) + O2 (g) 2 CaO
How many grams of calcium are required to completely react with 4.48 liters of O2 at STP?

42. Another Mixed Example 2 Ca (s) + O2 (g) 2 CaO
How many grams of calcium are required to completely react with 4.48 liters of O2 at STP?
X Grams Ca
4.48 L O2
2 mole Ca x g/mole Ca
1 mole O2 x 22.4 L/mole O2
X = g Ca

43. Other Possible Mixing
Sometimes you may start with moles and go to mass or liters
How will THAT work in the proportion?
If you are dealing with MOLES for a particular compound, simply leave out the second number on the bottom.

44. Mole-Mass Example
How many grams of iron could be made from 6 moles of carbon?

45. Mole-Mass Example
How many grams of iron could be made from 6 moles of carbon?
Grams Fe
6 moles C
2 mole Fe x g Fe
3 mole C
Grams Fe = g Fe
Notice that since we’re only
dealing with moles, you don’t
need a second number on the bottom!

46. Volume – Mole Example
How many moles of B5H9 are required to react with 100 L of O2?

47. Volume – Mole Example
Moles B5H9
100 L O2
How many moles of B5H9 are required to react with 100 L of O2?
2 moles B5H9
12 x 22.4 L
Notice that since we’re only
dealing with moles, you don’t
need a second number on the bottom!
Moles B5H9 = 0.74 moles

48. Percent Yield Theoretical Yield Actual Yield
The amount of material you can make from a reaction if it works perfectly….in theory
Everything you have calculated so far has been a theoretical yield
Yes, you’ve been doing THIS already
Actual Yield
The amount of stuff you actually make in a reaction

49. Percent Yield Percent Yield
Notice (again) that we multiply by 100 to turn a decimal to a percentage
The percent yield should NEVER be > 100%
If this occurs, you either made a math error or your sample is impure (wet).

50. Percent Yield Example (Easy)
Some problems will give you a theoretical and percent yield
A student produces 50 g of NaCl in a reaction yield where the theoretical yield is 60 g

51. Percent Yield Example (Easy)
Some problems will give you a theoretical and percent yield
A student produces 50 g of NaCl in a reaction yield where the theoretical yield is 60 g
50 g
60 g
x 100 = 83.3%

52. Percent Yield Example (Not As Easy…)
Calculate theoretical yield first
Then, plug into actual/theoretical x 100

53. Percent Yield Example (Not As Easy…)
X Grams KCl
45.8 g K2CO3
2 mole KCl x g/mole KCl
1 mole K2CO3 x g/mole K2CO3
46.3 g
Theoretical Yield = 49.5 g KCl
49.5 g
x 100 = 93.5 %

54. 3rd Percent Yield Example
2 Al(s) + 3 Cl2 (g) AlCl3 (s)
40.5 g of aluminum is reacted with excess chlorine gas to produce g of aluminum chloride. Give the percent yield of this process.

55. 3rd Percent Yield Example
2 Al(s) + 3 Cl2 (g) AlCl3 (s)
40.5 g ?????
actual yield = g
40.5 g of aluminum is reacted with excess chlorine gas to produce g of aluminum chloride. Give the percent yield of this process.

56. 3rd Percent Yield Example
2 Al(s) + 3 Cl2 (g) AlCl3 (s)
40.5 g ?????
actual yield = g
40.5 g of aluminum is reacted with excess chlorine gas to produce g of aluminum chloride. Give the percent yield of this process.
X Grams AlCl3
40.5 g Al
2 moles AlCl3 x g/mole AlCl3
2 mole Al x g/mole Al
180.0 g
Theoretical Yield = g AlCl3
200.1 g
x 100 = 90.0 %

57. Limiting Reagents We previously compared chemical reactions to recipes
If we’re lacking one ingredient, we can’t make the entire recipe.
This is called the limiting reagent (material that controls how much material we can make)
Example: we have 4 hamburgers, 4 slices of cheese, and 3 buns. We’re limited by the number of buns – this is our limiting reagent
We have an excess number of burgers and cheese slices

58. Limiting Reagents Non-chemical examples

59. Limiting Reagents

60. Limiting Reagents
C3H8 + 5 O2 3 CO2 + 4 H2O How many moles of carbon dioxide can be made from 2.5 moles of propane and 10 moles of oxygen? Notice that you are given amounts for BOTH reactants!

61. Limiting Reagents
C3H8 + 5 O2 3 CO2 + 4 H2O 2.5 moles 10 moles ?????? How many moles of carbon dioxide can be made from 2.5 moles of propane and 10 moles of oxygen? Find the moles of CO2 from both reactants!

62. Limiting Reagents
C3H8 + 5 O2 3 CO2 + 4 H2O 2.5 moles 10 moles ?????? How many moles of carbon dioxide can be made from 2.5 moles of propane and 10 moles of oxygen?
2.5 moles C3H8
????
From C3H8
1 mole C3H8 =
3 moles CO2
??? = 7.5 moles CO2
10 moles O2
????
From O2
5 moles O2 =
3 moles CO2
??? = 6 moles CO2

63. Limiting Reagents
C3H8 + 5 O2 3 CO2 + 4 H2O 2.5 moles 10 moles ?????? How many moles of carbon dioxide can be made from 2.5 moles of propane and 10 moles of oxygen?
2.5 moles C3H8
????
From C3H8
1 mole C3H8 =
3 moles CO2
??? = 7.5 moles CO2
10 moles O2
????
From O2
5 moles O2 =
3 moles CO2
??? = 6 moles CO2
Limiting reagent
Gave less product!

64. Limiting Reagents Mass-Mass Problem
CaO (s) + CO2 (g) CaCO3(s) How many grams of calcium carbonate are produced from 2.8 g calcium oxide and 4.48 L carbon dioxide at STP?
2.8 g CaO
4.48 L CO2
??????
X Grams CaCO3
2.8 g CaO
1 mole CaO x g/mole CaO
1 moles CaCO3 x g/mole CaCO3
X = 5.00 g CaCO3
X Grams CaCO3
4.48 L CO2
1 mole CO2 x 22.4 L/mole CO2
1 moles CaCO3 x g/mole CaCO3
X = g CaCO3

65. Terms To Know, Skills To Master
Mole Ratio
Percent Yield
Limiting Reagent
Skills to master
Calculate mole ratios
Perform mole-mole, mass-mass, mixed stoichiometry calculations
Determine percent yield from a reaction
Determine the limiting reagent of a reaction