Chem 1 unit 10 Stoichiometry

1. Stoichiometry
CHEMISTRY CHAPTER 11

2. Main Ideas
 The amount of each reactant present at the start of a
chemical reaction determines how much product can
form.
 The solution to every stoichiometric problem
requires a balanced chemical equation.
 Percent yield is a measure of the efficiency of a
chemical reaction.

3. 10.1 Defining Stoichiometry:
Objectives
•Describe the types of relationships
indicated by a balanced chemical
equation.
•State the mole ratios from a balanced
chemical equation.

4. Stoichiometry
Stoichiometry is the study of quantitative relationships
between the amounts of products formed by a
chemical reaction.
 Based on the law of conservation of mass.
 Mass of the reactants must equal the mass of the
products.
 Molecules are not necessarily balanced on each side.
Atoms, moles and mass do.

5. Practice Problem #1
C3H8(g) + 5O2(g)  3CO2 + 4H2O(g)
Moles:
Mass:
Molecules are not necessarily balanced on each side. Atoms, moles and mass do.

6. Mole Ratio
Mole ratio is a ratio between the numbers of moles of
any two substances in a balanced chemical equation.
 The number of mole ratios that can be written for
any equation is (n)x(n-1) where n is the number of
substances in the chemical reaction.

7. Practice Problem #2
4Al(s) + 3O2(g)  2Al2O3(s)
Mole Ratios:
3Fe(s) + 4H2O(l)  Fe3O4 + 4H2(g)
Mole Ratios:

8. Practice Problem #2
2HgO(s)  2Hg(l) + O2(g)
Mole Ratios:

9. Question
Which of the following is a correct mole ratio for the
following equation?
2Al(s) + 3Br2(l) → 2AlBr3(s)
A. 2 mol Al : 3 mol Br
B. 3 mol Br2 : 2 mol Al
C. 2 mol AlBr3 : 1 mol Br2
D. 2 mol Br : 2 mol Al

10. Question
How many mole ratios can be written for the following
reaction?
4H2(g) + O2(g) → 2H2O(l)
A. 6
B. 4
C. 3
D. 2

11. Practice Problems
 Page 371 #1 and 2
 Page 372 #4,5,6,9

12. 10.2 Stoichiometric Calculations
Objectives
•Listthe sequence of steps used in
solving stoichiometric problems.
•Solve stoichiometric problems.

13. Steps to Solving Stoichiometric Problems
1. Write a balanced equation
2. Determine the starting point of the calculations.
1. If units are mass units, convert to moles.
3. Use the correct reaction molar ratio to convert to
moles of substance in question.
4. Convert moles to requested units if needed.

14. Steps to Solving Stoichiometric Problems

15. Practice Problem #3: Moles to Moles
When burning propane (C3H8), carbon dioxide is
produced. How many moles of CO2 are produced
when 10.0 mol of C3H8 are burned in excess oxygen?

16. Practice Problem #3: Moles to Moles
Calculate the moles of CS2 and H2S produced when 1.5
mol S8 is used?
CH4(g) + S8(s)  CS2(l) + H2S(g)

17. Practice Problem #3: Moles to Moles
Sulfuric acid (H2SO4) is formed when sulfur dioxide
(SO2) reacts with oxygen and water. How many
moles of H2SO4 are produced from 12.5 moles of
SO2? How many moles of O2 are needed?

18. Practice Problem #4: Moles to Mass
Sodium chloride is decomposed into the elements
sodium and chlorine. How much chlorine gas, in
grams is obtained when 2.50 mol of sodium choride
are used?

19. Practice Problem #5: Mass to Mass
Acid rain (H2SO4) is formed from SO2, O2 and H2O. If
2.5g of SO2 reacts with excess oxygen and water, how
much H2SO4 is produced?

20. Questions
How many moles of CO2 will be produced in the
following reaction if the initial amount of reactants
was 0.50 moles?
2NaHCO3 → Na2CO + CO2 + H2O
A. 0.25
B. 0.3
C. 0.5
D. 1.0

21. Questions
A chemical reaction equation must be ____ in order
to make stoichiometric calculations.
A. measured
B. controlled
C. balanced
D. produced

22. Practice Problems
 Page 393 #61-63

23. 10.3 Limiting Reactant
Objectives
•Identify the limiting reactant in a
chemical equation.
•Identify the excess reactant, and calculate
the amount remaining after the reaction is
complete.
•Calculate the mass of a product when the
amounts of more than one reactant are
given.

24. Limiting Reactant
The limiting reactant limits the extent of the
reaction.
 Determine the amount of product formed.
 A portion of all the other reactants remains after the
reaction stops.
Excess reactants are the reactants leftover when a
reaction stops.

25. Example
Supplies First Aid Kits
 500 bandaids  24 bandaids
 85 gauze rolls  4 gauze rolls
 24 first aid ointments  1 first aid ointment
 24 burn sprays  1 burn spray
 65 rolls of tape  3 rolls of tape
 50 ace bandages  2 ace bandages

26. Steps to Determining the Limiting Reactant
1. Determine moles of reactants.
2. Use mole ratios to determine which reactant is
limiting.
3. Analyze the excess reactant
1. Moles reacted
2. Mass reacted
3. Excess remaining

27. Practice Problem #6
If 200.0g of sulfur reacts with 100.0g of chlorine, what
mass of disulfur dichloride is produced?
S8(l) + 4Cl2(g)  4S2Cl2(l)

28. Questions
The mass of the final product in a chemical reaction is
based on what?
A. the amount of excess reactant
B. the amount of limiting reactant
C. the presence of a catalyst
D. the amount of O2 present

29. Questions
What is the excess reactant in the following reaction if
you start with 50.0g of each reactant?
P4(s) + 5O2(g) → P4O10(s)
A. O2
B. P4
C. Both are equal.
D. unable to determine

30. Practice Problems
 page 383 #23-24

31. 10.4 Percent Yield
Objectives
•Calculatethe theoretical yield of a
chemical reaction from data.
•Determine the percent yield for a
chemical equation.

32. Theoretical Yield
Theoretical yield is the maximum amount of product
that can be produced from a given amount of
reactant.
 Assumes that the reaction goes to completion in the
forward direction.

33. Actual Yield
Actual yield is the amount of product produced when
the chemical reaction is carried out in an
experiment.

34. Percent Yield
Percent yield is the ratio of the actual yield expressed
as a percent.
 Represents how efficient a reaction is in producing
the desired product
 Percent yield = (actual yield/theoretical yield) x 100
actual yield(exp erimental)
%yield = x100%
theoretical yield(calculated)

35. Practice Problem #7
Solid silver chromate (AgCrO4) forms when potassium
chromate (K2CrO4) is added to a solution containing
.500g of silver nitrate (AgNO3). Determine the
theoretical yield of Ag2CrO4. Calculate the percent
yield if the reaction yields .455g of Ag2CrO4.

36. Practice Problem #7
AgNO3 + K2CrO4  Ag2CrO4 + ;
.500g AgNO3, theoretical yield of Ag2CrO4?
percent yield? .455g of Ag2CrO4

37. Questions
The amount of product that can be produced from a
given amount of reactants based on stoichiometric
calculations is:
A. actual yield
B. percent yield
C. theoretical yield
D. stoichiometric yield

38. Questions
You calculate the theoretical yield of a chemical
reaction starting with 50.0g of reactant is 25.0g of
product. What is the percent yield if the actual yield
is 22.0g of product?
A. 88%
B. 44%
C. 50%
D. 97%

39. Practice Problems
 page 387 #28-30

40. 10.4 Accumulating Content
•Apply knowledge and skills from
previous units to content learned in
this unit.

41. Practice Problem #8
When an antacid tablet dissolves in water, the fizz is
due to a reaction between sodium hydrogen carbonate
(baking soda), also called sodium bicarbonate, and
citric acid (H3C6H5O7). What are the masses of the
products if there is 1.00 gram of each reactant in the
tablet. Use the net ionic equation to solve this
problem.
What is the percent yield if only 1.25g of gas was
produced? How does this affect the resulting mass of
your other product?

42. Practice Problem #8
sodium bicarbonate, and citric acid (H3C6H5O7).
What are the masses of the products if there is 1.00
gram of each reactant in the tablet.
Use the net ionic equation to solve this problem.

43. Practice Problem #8
What is the percent yield if only 1.25g of gas was
produced? How does this affect the resulting mass of
your other product?

44. Study Guide
KEY CONCEPTS

45. Key Concepts
 Balanced chemical equations can be interpreted in
terms of moles, mass, and representative particles
(atoms, molecules, formula units).
 The law of conservation of mass applies to all
chemical reactions.
 Mole ratios are derived from the coefficients of a
balanced chemical equation. Each mole ratio
relates the number of moles of one reactant or
product to the number of moles of another
reactant or product in the chemical reaction.

46. Key Concepts
 Chemists use stoichiometric calculations to predict
the amounts of reactants used and products
formed in specific reactions.
 The first step in solving stoichiometric problems is
writing the balanced chemical equation.
 Mole ratios derived from the balanced chemical
equation are used in stoichiometric calculations.
 Stoichiometric problems make use of mole ratios
to convert between mass and moles.

47. Key Concepts
 The limiting reactant is the reactant that is completely
consumed during a chemical reaction. Reactants that
remain after the reaction stops are called excess
reactants.
 To determine the limiting reactant, the actual mole
ratio of the available reactants must be compared with
the ratio of the reactants obtained from the
coefficients in the balanced chemical equation.
 Stoichiometric calculations must be based on the
limiting reactant.

48. Key Concepts
 The theoretical yield of a chemical reaction is the
maximum amount of product that can be
produced from a given amount of reactant.
Theoretical yield is calculated from the balanced
chemical equation.
 The actual yield is the amount of product
produced. Actual yield must be obtained through
experimentation

49. Key Concepts
 Percent yield is the ratio of actual yield to
theoretical yield expressed as a percent. High
percent yield is important in reducing the cost of
every product produced through chemical
processes.

50. Questions
What law are all stoichiometric calculations based on?
A. law of definite proportions
B. law of conservation of mass
C. law of conservation of energy
D. none of the above

51. Questions
The mole ratios can be determined only if what?
A. all the reactants are present in
equal amounts
B. the reactants do not have
coefficients
C. the products do not have
coefficients
D. the equation is balanced

52. Questions
If the following reaction yields 5 mol NaAu(CN)2, how
many moles of Au were present as reactants?
(Assume all other reactants are in excess).
4Au(s) + 8NaCN(aq) + O2 + 2H2O(l) →
4NaAu(CN)2(aq) + 4NaOH(aq)
A. 1
B. 4
C. 5
D. 20

53. Questions
In the following reaction, how many moles of NaCN
are required to react with
5 mol of Au?
4Au(s) + 8NaCN(aq) + O2 + 2H2O(l) →
4NaAu(CN)2(aq) + 4NaOH(aq)
A. 3
B. 5
C. 8
D. 10

54. Questions
In the following reaction, what mass of NaOH is
produced if 5.0 moles of NaAu are also produced in
the reaction?
4Au(s) + 8NaCN(aq) + O2 + 2H2O(l) →
4NaAu(CN)2(aq) + 4NaOH(aq)
A. 20 g
B. 50 g
C. 200 g
D. 400 g

55. Questions
The SI base unit of amount is ____.
A. the gram
B. the kilogram
C. the mole
D. Avogadro’s number

56. Questions
Zinc reacts with iodine in a synthesis reaction: Zn + I2
 Znl2. What is the theoretical yield of Znl2, if 1.912
mol of zinc is used?
A. 6.103 g
B. 61.03 g
C. 610.3 g
D. 0.6103 g

57. Questions
In a chemical reaction, the statement that matter is
neither created nor destroyed is based on what?
A. mole ratio
B. law of conservation of mass
C. Avogadro’s number
D. law of definite proportions

58. Questions
Which is not a product that must be produced in a
double replacement reaction?
A. water
B. heat
C. precipitates
D. gases

59. Questions
The ____ is the maximum amount of product that can
be produced from a given amount of reactant.
A. theoretical yield
B. actual yield
C. limiting reactant
D. excess reactant