Stoichhhhhhhhhhhhhhiometry_Intro 2014.ppt

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1. Stoichiometry
© D Scott; CHS

2. StoichiometryStoichiometry
Consider the chemical equation:
4NH4NH
33 + 5O + 5O
22  6H 6H
22O + 4NOO + 4NO
There are several numbers involved.
What do they all mean?
““stochio” = Greek for elementstochio” = Greek for element
““metry” = measurementmetry” = measurement
  
Stoichiometry is about measuring the amounts of Stoichiometry is about measuring the amounts of
elements and compounds involved in a reaction. elements and compounds involved in a reaction.
© D Scott; CHS

3. StoichiometryStoichiometry
Recall that Chemical formulas represent numbers of atoms
4NH4NH
33 + 5O + 5O
22  6H 6H
22O + 4NOO + 4NO
NH
3 1 nitrogen and 3 hydrogen atoms
O
2
2 oxygen atoms
H
2
O2 hydrogen atoms and 1 oxygen atom
NO 1 nitrogen atom and 1 oxygen atom
© D Scott; CHS

4. StoichiometryStoichiometry
Recall that Chemical formulas are balanced with coefficients
4NH4NH
33 + 5O + 5O
22  6H 6H
22O + 4NOO + 4NO
4 X NH
3 = 4 nitrogen + 12 hydrogen
5 X O
2 = 10 oxygen
6 X H
2
O = 12 hydrogen + 6 oxygen
4 X NO = 4 nitrogen + 4 oxygen
© D Scott; CHS

5. StoichiometryStoichiometry
With Stoichiometry we find out that
4 : 5 : 6 : 4
do more than just multiply atoms.
4NH4NH
33 + 5O + 5O
22  6H 6H
22O + 4NOO + 4NO
© D Scott; CHS

6. StoichiometryStoichiometry
4NH4NH
33 + 5O + 5O
22  6H 6H
22O + 4NOO + 4NO
4 : 5 : 6 : 4
Can mean either:
4 molecules of NH
3 react with 5 molecules of O
2
to produce 6 molecules of H
2O and 4 molecules of NO
OR
4 moles of NH
3
react with 5 moles of O
2

to produce 6 moles of H
2O and 4 moles of NO
© D Scott; CHS
4 : 5 : 6 : 4
is what we call a mole ratio.

7. 4NH
3 + 5O
2  6H
2O + 4NO
How many moles of H
2
O are produced if 2.00
moles of O
2 are used?
Stoichiometry Question (1)
2.00 mol O
2
2.40 mol H
2
O=
Notice that a correctly balanced equation is
essential to get the right answer
6 mol H
2O
5 mol O
2
© D Scott; CHS

8. 4 mol NO
6 mol H
2
O
4 NH
3
+ 5 O
2
 6 H
2
O + 4 NO
How many moles of NO are produced in the
reaction if 15 mol of H
2O are also produced?
Stoichiometry Question (2)
15 mol H
2
O 10. mol NO=
© D Scott; CHS

10. 4 NH
3 + 5 O
2  6 H
2O + 4 NO
MOLE RATIO
(coefficients)

11. StoichiometryStoichiometry
Recall that Chemical formulas have molar masses:
4NH4NH
33 + 5O + 5O
22  6H 6H
22O + 4NOO + 4NO
NH
3 17 g/mol
O
2 32 g/mol
H
2O 18 g/mol
NO 30 g/mol
© D Scott; CHS

12. 4 NH
3
+ 5 O
2
 6 H
2
O + 4 NO
How many grams of H
2O are produced if 2.2 mol
of NH
3
are combined with excess oxygen?
Stoichiometry Question (3)
© D Scott; CHS
MOLE RATIO
(coefficients)
(H
2
O)(NH
3
)

13. 18.02 g H
2O
1 mol H
2
O
6 mol H
2O
4 mol NH
3
4 NH
3
+ 5 O
2
 6 H
2
O + 4 NO
How many grams of H
2O are produced if 2.2 mol
of NH
3
are combined with excess oxygen?
Stoichiometry Question (3)
2.2 mol NH
3

59 g
H
2
O
=
© D Scott; CHS

14. 5 mol O
2
6 mol H
2O
32 g O
2
1 mol O
2
4 NH
3 + 5 O
2  6 H
2O + 4 NO
How many grams of O
2 are required to produce
0.3 mol of H
2
O?
Stoichiometry Question (4)
0.3 mol H
2O 8 g O
2
=
© D Scott; CHS

15. 4 NH
3 + 5 O
2  6 H
2O + 4 NO
How many grams of NO are produced if 12 g of
O
2 is combined with excess ammonia?
Stoichiometry Question (5)
© D Scott; CHS
MOLE RATIO
(coefficients)
(NO)(O
2
)

16. 4 NH
3 + 5 O
2  6 H
2O + 4 NO
How many grams of NO are produced if 12 g of
O
2 is combined with excess ammonia?
4 mol NO
5 mol O
2
x
Stoichiometry Question (5)
12 g O
2
9.0 g NO
=
30.01 g NO
1 mol NO
x
1 mol O
2

32 g O
2
x
© D Scott; CHS

17. Moving along the stoichiometry path
We always use the same type of information to
make the jumps between steps:
grams (x)  moles (x)  moles (y)  grams (y)
Molar mass of x Molar mass of y
Mole ratio from
balanced equation
© D Scott; CHS

18. Many stoichiometry problems follow a pattern:
grams(x)  moles(x)  moles(y)  grams(y)
Converting grams to grams
We can start anywhere along this path
depending on the question we want to answer
Notice that we cannot directly convert from
grams of one compound to grams of another.
Instead we have to go through moles.
© D Scott; CHS

19. Have we learned it yet?
Try these on your own - 4 NH
3 + 5 O
2  6 H
2O + 4 NO
a) How many moles of H
2
O can be made using 1.6 mol NH
3
?
b) what mass of NH
3
is needed to make 0.75 mol NO?
c) how many grams of NO can be made from 47 g of NH
3
?
© D Scott; CHS

20. 4 NH
3 + 5 O
2  6 H
2O + 4 NO
a)
b)
c)
Answers
6 mol H
2O
4 mol NH
3
x 1.6 mol NH
3 2.4 mol
H
2O
=
4 mol NH
3
4 mol NO
x 0.75 mol NO 13 g
NH
3
=
17.04 g NH
3
1 mol NH
3
x
4 mol NO
4 mol NH
3
x 47 g NH
3

83 g NO
=
30.01 g NO
1 mol NO
x
1 mol NH
3
17.04 g NH
3
x
© D Scott; CHS