Stoichiometry

1
Stoichiometry

• Mathematical operations with chemical reactions
• Used to predict how much product will be produced
• Used to tell how much reactant will be needed

2
Remember - the mole (mol)

• The mole is a unit, just like the gram.
• It is used to measure the amount of substance
  present
• It is the units of the coefficients in a balanced
  equation
• 2H2 O2 ? 2H2O

3
Lets use the mole for our first stoichiometry
conversion

• Mole to mole stoichiometry problem
• A B ? C D
• Convert moles of one molecule to moles of another
• Problems that ask you to convert from moles of
  one substance to moles of a different substance

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Mole to mole flowchart
Moles of substance B The substance you Are
asked to end with

• Moles of substance A
• The substance you
• start with

5
Lets go through the steps with an example

• Equation
• H2 O2 ? H2O
• If four moles of hydrogen are used in this
  reaction, how many moles of oxygen are needed?

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Step 1

• Balance the equation
• ___H2 ___O2 ? ___H2O

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Step 2

• Write down what is given in the problem and put
  it over 1
• 4 moles H2
• -------------------
• 1

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Step 3 _2_H2 ___O2 ? _2__H2O

• Use your coefficients from the balanced equation
  as a conversion factor.
• Called the mole ratio
• Be sure to include units and make sure that your
  units cancel.

4 moles H2 ------------------- x
------------------ 1
9
Step 4

• Solve the problem mathematically

4 moles H2 1 mol
O2 ------------------- x ------------------
1 2 moles H2
2 moles O2
10
You try an example

• Use the same balanced equation
• How many mols of oxygen must have been used if 14
  mols of water were produced?

11
Bell Work

• In the reaction shown here, how many moles of
  iron are needed to react completely with 32.0
  moles of sulfur? (This is a mole to mole
  conversion problem follow the example in your
  notes from yesterday)
• Fe S ? FeS

12
Review problem

• When zinc reacts with sulfuric acid, how many
  moles of hydrogen are produced from 31.8 moles of
  zinc?
• Zn H2SO4 ? ZnSO4 H2

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Another review

• How much sulfur dioxide is needed to react when
  128 g of sulfic acid was produced?
• SO2 H2O ? H2SO3

14
More review

• Given the following equation
• 2 C4H10 13 O2 ---gt 8 CO2 10 H2O
• How many moles of oxygen are needed to produce 5
  moles of water?

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One more

• Glucose is used as a source of energy by the
  human body. The overall reaction in the body is
• C6H12O6 O2 ? CO2 H2O
• Calculate the number of moles of oxygen needed to
  oxidize (react with) 12.5 moles of glucose to
  carbon dioxide and water.

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Lets go on to our second type of stoichiometry
problem

• Before we do, we need to review vocabulary term
• Molar mass

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Molar mass

• Also known as gram formula mass
• Mass of a molecule
• Tells us the mass of one mole of each molecule.
• Steps
• Multiply the subscript for each element by the
  mass of each element and then add the answers
  together

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Mass to mol stoichiometry

• In this problem, you are given grams to start
  with not mols. Then you are asked to find the
  mols of a different substance in the chemical
  reaction.

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Mass to mole flowchart

• Grams of
• substance A
• The substance
• you start with

Moles of substance B The substance you Are
asked to end with

• Moles of substance A
• The substance you
• Start with

20
Lets go through the steps with an example

• Equation
• H2 O2 ? H2O
• If 28 grams of hydrogen are used in this
  reaction, how many moles of oxygen are needed?

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Step 1

• Balance the equation
• ___H2 ___O2 ? ___H2O

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Step 2

• Write down what is given in the problem and put
  it over 1
• 28 grams H2
• -------------------
• 1

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New step 3

• Since we are not given mols, we have to convert
  from grams to mols using molar mass
• 28 grams H2
• ------------------- x ---------------
• 1
• Now we use this answer to complete the rest of
  the problem, just like we did in the mol to mol
  conversion

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Step 4

• Write down how many mols you have and put it over
  1
• 14 mols H2
• -------------------
• 1

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Step 5 _2_H2 ___O2 ? _2__H2O

• Use your coefficients from the balanced equation
  as a conversion factor.
• Called the mole ratio
• Be sure to include units and make sure that your
  units cancel.

14 moles H2 ------------------- x
------------------ 1
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Step 6

• Solve the problem mathematically

14 moles H2 1 mol
O2 ------------------- x ------------------
1 2 moles H2
7 moles O2
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You try an example

• Use the same balanced equation
• How many mols of oxygen must have been used if
  140 grams of water were produced?

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• Ammonia is synthesized from hydrogen and nitrogen
  according to the following equation.
• N2 (g) 3H2 (g) ? 2NH3 (g)
• If 3.41 grams of hydrogen react with nitrogen,
  how many moles of ammonia are produced?

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• How many moles of hydrogen gas are formed when
  2.33 grams of lithium reacts with water?
• 2Li (aq) 2H2O (l) ? 2LiOH (aq) H2 (g)

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• In the reaction shown here, how many moles of
  iron is needed to react completely with 32.0 g of
  sulfur?
• Fe S ? FeS

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Mol to mass stoichiometry

• In this problem, you are given mols to start
  with. Then you are asked to find the grams of a
  different substance in the chemical reaction.

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Mole to mass flowchart

• Grams of
• substance B
• The substance
• you end with

Moles of substance B The substance you Are
asked to end with

• Moles of substance A
• The substance you
• Are asked to start with

33
Lets go through the steps with an example

• Equation
• H2 O2 ? H2O
• If 4 mols of hydrogen are used in this reaction,
  how many grams of oxygen are needed?

34
Step 1

• Balance the equation
• ___H2 ___O2 ? ___H2O

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Step 2

• Write down what is given in the problem and put
  it over 1
• 4 mols H2
• -------------------
• 1

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Step 3

• Write down how many mols you have and put it over
  1
• 4 mols H2
• -------------------
• 1

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Step 4 _2_H2 ___O2 ? _2__H2O

• Use your coefficients from the balanced equation
  as a conversion factor.
• Called the mole ratio
• Be sure to include units and make sure that your
  units cancel.

4 moles H2 ------------------- x
------------------ 1
38
Step 4

• Solve the problem mathematically for mols

4 moles H2 1 mol
O2 ------------------- x ------------------
1 2 moles H2
2 moles O2
39
Step 5

• We are not done yet since our answer is in
  moles, we have one more conversion to do to get
  to grams
• Write down how many mols you have over 1
• 2 mols O2
• ---------------- We need to use molar mass
• 1 to get to grams

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Step 6

• Convert from mols to grams
• 2 mols O2
• ---------------- x -----------
• 1

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You try an example

• Use the same balanced equation
• How many grams of oxygen must have been used if
  14 mols of water were produced?

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Mass to mass stoichiometry

• Mass to mass stoichiometry
• Given grams of one substance and asked to solve
  for grams of another substance

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Mass to mass flowchart

• Grams of
• substance A
• The substance
• you start with

• Grams of
• substance B
• The substance
• you end with

Moles of substance B The substance you Are
asked to end with

• Moles of substance A
• The substance you
• Are asked to start with

44
Lets go through the steps with an example

• Equation
• H2 O2 ? H2O
• If 48 grams of hydrogen are used in this
  reaction, how many grams of oxygen are needed?

45
Step 1

• Balance the equation
• ___H2 ___O2 ? ___H2O

46
Step 2

• Write down what is given in the problem and put
  it over 1
• 48 grams H2
• -------------------
• 1

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Step 3

• Since we are not given mols, we have to convert
  from grams to mols using molar mass
• 48 grams H2
• ------------------- x ---------------
• 1
• Now we use this answer to complete the rest of
  the problem, just like we did in the mol to mol
  conversion

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Step 4

• Write down how many mols you have and put it over
  1
• 24 mols H2
• -------------------
• 1

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Step 5 _2_H2 ___O2 ? _2__H2O

• Use your coefficients from the balanced equation
  as a conversion factor.
• Called the mole ratio
• Be sure to include units and make sure that your
  units cancel.

24 moles H2 ------------------- x
------------------ 1
50
Step 6

• Solve the problem mathematically for mols

24 moles H2 1 mol
O2 ------------------- x ------------------
1 2 moles H2
12 moles O2
51
Step 7

• We are not done yet since our answer is in
  moles, we have one more conversion to do to get
  to grams
• Write down how many mols you have over 1
• 12 mols O2
• ---------------- We need to use
  molar mass
• 1 to get
  to grams

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Step 8

• Convert from mols to grams
• 12 mols O2
• ---------------- x -----------
• 1

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You try an example

• Use the same balanced equation
• How many grams of oxygen must have been used if
  248 grams of water were produced?

54
Bell Work

• 8 Fe S8 ---gt 8 FeS
• If you have 100 grams of Fe, how many grams of
  FeS are produced?

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Bell Work

• Predict the products and balance the following
• Na Pb2SO4 ?
• If 50 grams of Na are used in the reaction, how
  many grams of Pb are produced?

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Percent Yield

• Tells us how much of the product you actually
  produced in the lab
• Formula Experimental value
• ------------------------- x
  100
• Theoretical value

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Bell Work

• Pb(NO3)2 ? Pb NO2 O2
• Balance the equation
• What type of reaction is this?
• If you have 50 grams of lead (II) nitrate, how
  many grams of nitrogen dioxide can be produced?

58
Bell Work

• Magnesium sulfide reacts with water.
• Write a complete balanced equation for this
  reaction.
• If 100.0 grams of water are used in the reaction,
  how many grams of Magnesium oxide are formed?