Stoichiometry
Chemistry
Dr. May
Moles
• One mole of anything contains Avogadro’s
number of particles:
• One mole of sodium (Na) contains
6.02 x 1023 atoms with a mass of 23 grams
• One mole of water (H2O) contains
• 6.02 x 1023 molecules and a mass of 18 grams
Molar Mass (Molecular Weight) (MW)
• Molar mass is the sum of the atomic masses
of all the atoms in a molecule
•
•
•
•
The
The
The
The
molar
molar
molar
molar
mass of
mass of
mass of
mass of
H2O is (2+16) = 18 grams
CH4 (methane) is (12 + 4) = 16 grams
CO2 is (12 + 32) = 44 grams
N2 is (14 + 14) = 28 grams
• 1 molar mass = 6.02 x 1023 molecules = 1 mole
Molar Ratio
• Zn
1
+ 2 HCl 
ZnCl2
+
H2
The molar ratio is:
:
2
:
1
:
1
• Or, 1 mole of zinc plus 2 moles of
hydrochloric acid yields 1 mole of zinc
chloride plus 1 mole of hydrogen gas
Molar Ratio And Moles
• Zn
1
•
•
•
•
1
2
1
1
+ 2 HCl 
:
2
:
ZnCl2
1
+
:
H2
1
mole of Zn has a mass of 65 grams
moles of HCl has a mass of 72 grams
mole of ZnCl2 has a mass of 135 grams
mole of H2 has a mass of 2 grams
Molar Ratio And Moles
• Zn
+ 2 HCl 
ZnCl2
+
H2
• 1 mole of Zn has a mass of 65 grams
• 2 moles of HCl has a mass of 72 grams
137 grams
• 1 mole of ZnCl2 has a mass of 135 grams
• 1 mole of H2 has a mass of _ 2 grams
137 grams
Conservation Of Mass
• In a balanced chemical equation, the
total formula mass of the reactants
must always equal the total formula
mass of the products
Moles And Mass
Moles = Grams
MW
Grams = Moles x MW
Mole-Mass Relationship
Grams = Moles x MW
• What is the mass of
1 mole of N2 ?
• 2 x 14 = 28 grams
• What is the mass of
3 moles of H2 ?
• 3 x 2 = 6 grams
Mole-Mole Relationships
3 H2
mr:
•
•
•
•
3
1
2
6
3
+
N2

2 NH3
:
1
:
2
moles of H2 (6 grams) combines with
mole of N2 (28 grams) to give
moles of ammonia gas (34 grams)
grams + 28 grams = 34 grams
Mole-Mole Relationships
mr:
moles:
•
•
•
•
3 H2
3
6
+
:
N2
1
2

:
2 NH3
2
4
6 moles of H2 (12 grams) combines with
2 mole of N2 (56 grams) to give
2 moles of ammonia gas (68 grams)
12 grams + 56 grams = 68 grams
Mole-Mole Relationships
mr:
moles:
•
•
•
•
3 H2
3
1.5
+
:
N2
1
0.5

:
2 NH3
2
1
1.5 moles of H2 (3 grams) combines with
0.5 moles of N2 (14 grams) to give
1 mole of ammonia gas (17 grams)
3 grams + 14 grams = 17 grams
Mole-Mole Relationships
mr:
moles:
•
•
•
•
3 H2
+
N2

3
1
:
1
0.33
:
2 NH3
2
0.67
1 mole of H2 (2 grams) combines with
0.33 moles of N2 (9.3 grams) to give
0.67 moles of ammonia gas (11.3 grams)
2 grams + 9.3 grams = 11.3 grams
Mole-Mole Relationships
3 H2
+
N2

2 NH3
How many moles of nitrogen are
needed to react completely with 3
moles of hydrogen ?
The answer is …….. 1 mole
Mole-Mole Relationships
3 H2
+
N2

2 NH3
How many moles of nitrogen are
needed to react completely with 1.5
moles of hydrogen ?
The answer is …….. 0.5 moles
Mole-Mole Relationships
3 H2
+
N2

2 NH3
How many moles of nitrogen are
needed to react completely with 6
moles of hydrogen ?
The answer is …….. 2 moles
Mole-Mole Relationships
3 H2
+
N2

2 NH3
How many moles of ammonia do
you get when 1 mole of nitrogen is
reacted completely with 3 moles of
hydrogen ?
The answer is …….. 2 moles
Stoichiometry (The Matrix)
MW
2
Eq.
3 H2 +
MR
Moles
grams
28
17

N2
2 NH3
3
1
2
0.5
0.17
0.33
1.0
4.7
5.7
Mass-Mass Relationships
3 H2
+
N2

2 NH3
How many grams of nitrogen are
needed to react completely with 6
grams of hydrogen ?
The answer is …….. 28 grams
How did we know that ????
Stoichiometry
MW
2
Eq.
3 H2
MR
3
1
2
3.0
1.0
2.0
6.0
28.0
34.0
Moles
grams
28
+
17

N2
2 NH3
Mass-Mass Relationships
3 H2
+
N2

2 NH3
How many grams of nitrogen are
needed to react completely with 12
grams of hydrogen ?
The answer is …….. 56 grams
Mass-Mass Relationships
3 H2
+
N2

2 NH3
How many grams of nitrogen are
needed to react completely with 4.7
grams of hydrogen ?
The answer is not obvious !
Mass-Mass Relationships
• Shall we go to our STOICHIOMETRY
Matrix ????
• Away we go !!!!!!!
Stoichiometry
MW
2
Eq.
3 H2 +
MR
3
1
2
?
2.35
0.78
1.57
Grams =
21.9
moles
x MW
26.6
Moles
grams
Moles = grams
MW
4.7
28
17

N2
2 NH3
Real Stoichiometry (The Equation)
H2
+
N2

NH3
Real Stoichiometry (Balance Equation)
3 H2
+
N2

2 NH3
Real Stoichiometry (The Molar Ratio)
3 H2
3
+
N2
1

2 NH3
2
Real Stoichiometry (The Molar Mass)
2
3 H2
3
+
28
N2
1

17
2 NH3
2
Real Stoichiometry (The Problem)
2
3 H2
+
3
1.5 g.
28
N2
1

17
2 NH3
2
?
• How many grams of NH3 are made
when 1.5 grams H2 reacts completely
with N2 ?
Real Stoichiometry (Find Moles)
2
3 H2
3
Gr.
Moles
+
28
N2
1

1.5 g.
??
Moles = Grams
MW
Moles = 1.5  2 = 0.75
17
2 NH3
2
Real Stoichiometry (Move Moles)
2
3 H2
3
Gr.
Moles
+
28
N2
1

17
2 NH3
2
1.5 g.
0.75           
The molar ratio of NH3 to H2 is 2:3
The number of moles of NH3 is 2/3 of the moles of H2
2/3 of 0.75 = 0.50
?
Real Stoichiometry (Find Grams)
2
3 H2
3
Gr.
Moles
+
28
N2
1

17
2 NH3
2
1.5 g.
?
0.75             0.50
Grams = moles x MW = 0.50 x 17 = 8.5
Real Stoichiometry (The Answer)
2
3 H2
3
Gr.
Moles
+
28
N2
1

17
2 NH3
2
1.5 g.
8.5 g.
0.75             0.50
Grams = moles x MW = 0.50 x 17 = 8.5
Real Stoichiometry (The Solution))
2
3 H2
3
1.5 g.
28
+
N2
1

17
2 NH3
2
?
• How many grams of NH3 are made when 1.5 grams H2 reacts
completely with N2 ?
• The answer is 8.5 grams
The End
• This presentation was created for the
benefit of our students by the Science
Department at Howard High School of
Technology
• Please send suggestions and comments
to rmay@nccvt.k12.de.us