Stoichiometric Calculations

1
Stoichiometric Calculations

• Stoichiometry Ch. 11

2
Background on things you NEED to know how to do

1. Name/write correct chemical formula
2. Write chemical equations
3. Balance chemical equations
4. Predict Products
5. Mole/mass conversions

3
Stoichiometry

• Stoichiometry uses ratios to determine relative
  amounts of reactants or products.
• For example If you were to make a bicycle, you
  would need one frame and two tires.
• 1 frame 2 tires ? 1 bicycle
• If I had 74 tires, what is the most of bicycles
  I could make?

74 tires
1 bicycle
37 bicycles
2 tires
4
Proportional Relationships
3/4 c. brown sugar 1 tsp vanilla extract 2 eggs 2
c. chocolate chips Makes 5 dozen cookies.
2 1/4 c. flour 1 tsp. baking soda 1 tsp. salt 1
c. butter 3/4 c. sugar

• I have 5 eggs. How many cookies can I make?

5 eggs
5 doz. 2 eggs
12.5 dozen cookies
5
Proportional Relationships

• Stoichiometry
• mass relationships between substances in a
  chemical reaction
• based on the mole ratio
• Mole Ratio
• indicated by coefficients in a balanced equation
• can be used to determine expected amounts of
  products given amounts of reactants.

2 Mg O2 ? 2 MgO
6
Stoichiometry Steps

• 1. Write a balanced equation.
• 2. Identify known unknown.
• 3. Line up conversion factors.
• Mole ratio - moles ? moles
• Molar mass - moles ? grams
• Molarity - moles ? liters soln
• Molar volume - moles ? liters gas

• Mole ratio - moles ? moles

Core step in all stoichiometry problems!!
4. Check answer.
7
Molar Volume at STP
LITERS OF GAS AT STP
Molar Volume (22.4 L/mol)
MASS IN GRAMS
NUMBER OF PARTICLES
MOLES
Molar Mass (g/mol)
6.02 ? 1023 particles/mol
Molarity (mol/L)
LITERS OF SOLUTION
8
Mole Mole Conversions

• The first type of problems we encounter will go
  between moles and moles. For this we need to use
  mole ratios.
• Ex Write and balance the reaction between lead
  (II) nitrate and potasium iodide.

Pb(NO3)2 2KI ?
2 KNO3 PbI2
Mole ratio of potasium iodide to lead (II) iodide
2 moles KI
1 mole PbI2
9
Mole to Mole Problems

• How many moles of KClO3 must decompose in order
  to produce 9 moles of oxygen gas?

2KClO3 ? 2KCl 3O2
? mol
9 mol
9 mol O2
2 mol KClO3 3 mol O2
6 mol KClO3
10
Mole to Mass

• We can also convert from moles to mass, and mass
  to moles
• For Example
• 4 Al 3 O2 ? 2Al2O3
• If you know how many grams of Al you start with,
  we can write a flow chart to show how to
  calculate the of moles of oxygen need to fully
  react with the Al.
• g Al ? moles Al ? moles of oxygen

11
Mass to Moles 4 Al 3 O2 ? 2Al2O3

• If the reaction starts with .84 moles of
  aluminum, how many grams of aluminum oxide are
  produced?

2 mol Al2O3
.84 mol Al
101.9 grams Al2O3
4 mol Al
1 mol Al2O3
42.8 grams Al2O3

• 0.92 g of Aluminum oxide are produced from the
  reaction. How much aluminum was used up?

26.9 g Al
4 mol Al
1 mol Al2O3
.92 g Al2O3
1 mol Al
101.9 g Al2O3
2 mol Al2O3
.49 grams Al
12
Mass to Mass

• How many grams of silver will be formed from 12.0
  g copper reacting with silver nitrate?

Cu 2AgNO3 ? 2Ag Cu(NO3)2
12.0 g
? g
12.0 g Cu
1 mol Cu 63.55 g Cu
2 mol Ag 1 mol Cu
107.87 g Ag 1 mol Ag
40.7 g Ag
13
Stoichiometry Problems Mole-Mole

• N2H4 N2O4 ? N2 H2O
• ? moles N2O4 2.72 moles N2H4

4
2
3
2.72 mol
? mol
2.72 mol N2 H4
1 mol N2O4 2 mol N2 H4
1.36 mol N2O4
14
Stoichiometry Problems Mass/Mass

• Sodium metal reacts with oxygen gas to produce
  solid sodium oxide. How many grams of sodium must
  react to produce 42.0 grams of sodium oxide?

4Na O2 ? 2Na2O
? g
42.0 g
42.0 g Na2O
1 mol Na2O
22.99 g Na
4 mol Na
22.99 g Na
16.98 g Na2O
1 mol Na
2 mol Na2O
15
Stoichiometry Problems Mole/Mass

• In photosynthesis, carbon dioxide and water react
  to form glucose, C6H12O6 and oxygen gas.
• ___CO2 ___H2O ? ___C6H12O6 ___O2
• If 15.6 grams of carbon dioxide react, how many
  moles of glucose will be produced?
• How many grams of carbon dioxide must react to
  produce 0.25 moles of glucose?

6
6
6
15.6 g CO2
1 mol CO2
1 mol C2 H12O6
0.0591 mol C2H12O6
6 mol CO2
44.01 g CO2
0.25 mol C2 H12O6
44.01 g CO2
6 mol CO2
66 g CO2
1 mol C2 H12O6
1 mol CO2
16
Stoichiometry with Gases

• If the pressure and temperature are constant, the
  ratio of moles in the balanced equation is the
  ratio of liters in an all gas reaction.

17
Molar Volume at STP
At STP 1 mol of a gas22.4 L
18
Molar Volume

• Hydrogen and chlorine gas react to produce
  hydrochloric acid. If 7.00 L of hydrogen gas
  react, how many liters of HCl gas are formed?

H2 (g) Cl2 (g) ? 2 HCl (g)
2.0 L HCl
7.00 L H2
14.0 L HCl only in all gas reactions!
1.0 L H2
19
Molar Volume

• In the following reaction, if 17 g of Mg react,
  how many L of H2 forms?
• Mg (s) 2HCl (aq) ? MgCl2 (aq) H2 (g)

17.0 g Mg
1 mol Mg 24.31 g Mg
1 mol H2 1 mol Mg
22.4 L H2 1 mol H2
15.7 L H2
20
Molar Volume Problems

• How many grams of KClO3 are reqd to produce 9.00
  L of O2 at STP?

2KClO3 ? 2KCl 3O2
? g
9.00 L
9.00 L O2
1 mol O2 22.4 L O2
2 mol KClO3 3 mol O2
122.55 g KClO3 1 mol KClO3
32.8 g KClO3
21
Molar Volume Problems

• How many grams of Cu are required to react with
  1.5 L of 0.10M AgNO3?

Cu 2AgNO3 ? 2Ag Cu(NO3)2
1.5L 0.10M
? g
63.55 g Cu 1 mol Cu
1.5 L
.10 mol AgNO3 1 L
1 mol Cu 2 mol AgNO3
4.8 g Cu
22
Molarity

• Molarity is the number of moles of solute
  dissolved in one liter of solution.
• Units are moles per liter or moles of solute per
  liter of solution.
• Molarity abbreviated by a capital M
• Molarity moles of solute liter of
  solution

23
Molarity Problems

• As an example, suppose we dissolve 23 g of
  ammonium chloride (NH4Cl) in enough water to make
  145 mL of solution. What is the molarity of
  ammonium chloride in this solution?

1 mole NH4Cl
23 g NH4Cl
.43 mol NH4Cl
53.5 g NH4Cl
145 mL
1 L
.145 L
1000 mL
.43 mol NH4Cl

• 2.97 M NH4Cl

.145 L
24
Molarity Problems

• Now, suppose we have a beaker with 175 mL of a
  5.5 M HCl solution. How many moles of HCl is in
  this beaker?

5.5 mol HCl

• .96 mol HCl

175 mL
1 L
1000 mL
1 L

• Suppose you had 70 grams of NaCl and you
  dissolved it in exactly 2.00 L of solution. What
  would be the molarity of the solution?

1.2 mol NaCl
70 g NaCl
1 mol NaCl
2.0 L
1.2 mol NaCl
58.44 g NaCl
0.6 M NaCl
25
Volume of Solutions

• 
  
• A 10. HCl solution (soln) means
  10 g HCl
  (pure) 100
  g HCl soln
• 
  
• A solution with a density of 1.5 g/mL means
  1.5 g soln
  1 mL soln

26
Impure Substances

• 
  
• To say a substance is 75 NaCl by mass means
  
  75 g NaCl (pure)
  100 g of NaCl solution
• or
• 100 g of NaCl solution
• 75 g of NaCL
  (pure)
• Or, Iron ore that is 15 iron by mass means
  15 g Fe
  100 g ore
• or
  
• 100 g of ore
• 15 g of Fe

27
Energy Stoichiometry
28
Exothermic and Endothermic

• Exothermic process heat is released into the
  surroundings
• Exo Exit
• Endothermic Process heat is absorbed from the
  surroundings
• Endo Into

29
Thermochemical Equations

• In a thermochemical equation, the energy of
  change for the reaction can be written as either
  a reactant or a product
• Enthalpy the heat content of a system at
  constant pressure (?H)
• Endothermic (positive ?H)
• 2NaHCO3 129kJ Na2CO3 H2O CO2
• Exothermic (negative ?H)
• CaO H2O Ca(OH)2 65.2kJ

30

• Write the thermochemical equation for the
  oxidation of Iron (III) if its ?H -1652 kJ
• Fe(s) O2(g)? Fe2O3(s) 1652 kJ
• How much heat is evolved when 10.00g of Iron is
  reacted with excess oxygen?

Exo
4
3
2
1 mol Fe
10.00g Fe
1652 kJ
73.97 kJ of heat
4 mol Fe
55.85g Fe
31

• Write the thermochemical equation for the
  decomposition of sodium bicarbonate, with
  a ?H 129 kJ
• 2 NaHCO3 129kJ ? Na2CO3(s) H2O CO2
• How much heat is required to break down 50.0g of
  sodium bicarbonate?

Endo
1 mol NaHCO3
50.0 g NaHCO3
129 kJ
83.9 g NaHCO3
2 mol NaHCO3
38.4 kJ of heat
32

• Write the thermochemical equation for the
  synthesis of calcium oxide and water with a ?H
  - 65.2 kJ
• CaO H2O ? Ca(OH)2 65.2kJ
• How much energy is released when 100 g of
  calcium oxide reacts?

Exo
100 g CaO
1 mol CaO
65.2 kJ
56.07 g CaO
1 mol CaO
116 kJ of heat
33

• Write the thermochemical equation for the
  decomposition of magnesium oxide with a ?H
  61.5 kJ
• 2 MgO 61.5 ? 2 Mg O2
• How many grams of oxygen are produced when
  magnesium oxide is decomposed by adding 420 kJ of
  Energy?

Endo
420 kJ
1 mol O2
31.98 g O2
218 g of O2
61.5 kJ
1 mol O2
34
Stoichiometry in the Real World

• Stoichiometry Ch. 11

35
Limiting Reactants

• Available Ingredients
• 4 slices of bread
• 1 jar of peanut butter
• 1/2 jar of jelly

• Limiting Reactant
• bread

• Excess Reactants
• peanut butter and jelly

36
Limiting Reactants

• Available Ingredients
• 24 graham cracker squares
• 1 bag of marshmallows
• 12 pieces of chocolate

• Limiting Reactant
• chocolate

• Excess Reactants
• Marshmallows and graham crackers

37
Limiting Reactants

• Limiting Reactant
• one that is used up in a reaction
• determines the amount of product that can be
  produced
• Excess Reactant
• added to ensure that the other reactant is
  completely used up
• cheaper easier to recycle

38
Limiting Reactant Steps

• 1. Write the balanced equation.
• 2. For each reactant, calculate the amount of
  product formed.
• 3. Smaller answer indicates
• limiting reactant
• amount of product actually possible

39
Limiting Reactants

• 79.1 g of zinc react with 68.1 g HCl. Identify
  the limiting and excess reactants. How many
  grams of hydrogen can be formed?

Zn 2HCl ? ZnCl2 H2
? g
79.1 g
68.1 g
40
Limiting Reactants
Zn 2HCl ? ZnCl2 H2
? g
79.1 g
68.1 g
2.02 g H2 1 mol H2
68.1 g HCl
1 mol HCl 36.46 g HCl
1 mol H2 2 mol HCl
1.89 g H2
41
Limiting Reactants
Zn 2HCl ? ZnCl2 H2
? g
79.1 g
68.1 g
79.1 g Zn
1 mol Zn 65.39 g Zn
1 mol H2 1 mol Zn
2.02 g H2 1 mol H2
2.44 g H2
42
Limiting Reactants

• Zn 2.44 g H2 HCl 1.89 g H2

Limiting reactant HCl Excess reactant
Zn Product Formed 1.89 g H2
43
Limiting Reactants 2

• 5.42 g of magnesium ribbon react with 4.00 g of
  oxygen gas. Identify the limiting and excess
  reactants. How many grams of magnesium oxide are
  formed?

2Mg O2 ? 2MgO
4.00 g
5.42 g
? g
44
Limiting Reactants 2
2Mg O2 ? 2MgO
? g
4.00 g
5.42 g
5.42 g Mg
1 mol Mg 24.31 g Mg
2 mol MgO 2 mol Mg
40.31 g MgO 1 mol MgO
8.99 g MgO
45
Limiting Reactants 2
2Mg O2 ? 2MgO
? g
4.00 g
5.42 g
40.31 g MgO 1 mol MgO
4.00 g O2
1 mol O2 32.00 g O2
2 mol MgO 1 mol O2
10.1 g MgO
46
A. Limiting Reactants 2

• Mg 8.99 g MgO O2 10.1 g MgO

Limiting reactant Mg Excess reactant
O2 Product Formed 8.99 g MgO
47
Limiting Reactants

• What other information could you find in these
  problems?
• How much of each reactant is used in grams,
  liters, moles
• How much of excess reactant is left over in
  grams, liters, moles

48
Percent Yield
49
Percent Yield

• When 45.8 g of K2CO3 react with excess HCl, 46.3
  g of KCl are formed. Calculate the theoretical
  and yields of KCl.

K2CO3 2 HCl ? 2 KCl
H2CO3
1 mol K2CO3 138.2 g K2CO3
2 mol KCl 1 mol K2CO3
74.55 g KCl 1 mol KCl
45.8 g K2CO3
49.4 grams KCl
50
Percent Yield

• When 45.8 g of K2CO3 react with excess HCl, 46.3
  g of KCl are formed. Calculate the theoretical
  and yields of KCl.

46.3 grams KCl
x 100
49.4 grams KCl
yield 93.7