Stoichiometry
Lancaster High School
Stoichiometry
“stochio” = ___________
“metry” = ____________
Stoichiometry is about measuring the amounts of
elements and compounds involved in a reaction.
Consider the chemical equation:
4NH3 + 5O2  6H2O + 4NO
There are several numbers involved.
What do they all mean?
Stoichiometry
4NH3 + 5O2  6H2O + 4NO
Recall that Chemical formulas represent numbers of atoms
NH3
__ nitrogen and __ hydrogen atoms
O2
__ oxygen atoms
H 2O
__ hydrogen atoms and __ oxygen
atom
NO
__ nitrogen atom and __ oxygen atom
Stoichiometry
4NH3 + 5O2  6H2O + 4NO
Recall that Chemical formulas have molar masses:
NH3
__ g/mol
O2
__ g/mol
H 2O
__ g/mol
NO
__ g/mol
Stoichiometry
4NH3 + 5O2  6H2O + 4NO
Recall that Chemical formulas are balanced with coefficients
4 X NH3
= __ nitrogen + __ hydrogen
5 X O2
= __ oxygen
6 X H 2O
= __ hydrogen + __ oxygen
4 X NO
= __ nitrogen + __ oxygen
Stoichiometry
4NH3 + 5O2  6H2O + 4NO
With Stoichiometry we find out that
4:5:6:4
do more than just multiply atoms.
4:5:6:4
Are what we call a ____ ratio.
Stoichiometry
4NH3 + 5O2  6H2O + 4NO
4:5:6:4
Can mean either:
4 _________ of NH3 react with 5 _________ of O2
to produce 6 __________ of H2O and 4 _________ of NO
OR
4 ______ of NH3 react with 5 ______ of O2
to produce 6 ______ of H2O and 4 ______ of NO
Stoichiometry Question (1)
4NH3 + 5O2  6H2O + 4NO
 How
many moles of H2O are produced if 2.00
moles of O2 are used?
Notice that a correctly __________ equation is
essential to get the right answer
Stoichiometry Question (2)
4 NH3 + 5 O2  6 H2O + 4 NO
How many moles of NO are produced in the
reaction if 15 mol of H2O are also produced?
Stoichiometry Question (3)
4 NH3 + 5 O2  6 H2O + 4 NO
 How
many grams of H2O are produced if 2.2 mol
of NH3 are combined with excess oxygen?
Stoichiometry Question (4)
4 NH3 + 5 O2  6 H2O + 4 NO
 How
many grams of O2 are required to produce
0.3 mol of H2O?
Stoichiometry Question (5)
4 NH3 + 5 O2  6 H2O + 4 NO
 How
many grams of NO is produced if 12 g of O2
is combined with excess ammonia?
Moving along the stoichiometry path
 We
always use the same type of information to
make the jumps between steps:
_______of x
_________of y
grams (x)  moles (x)  moles (y)  grams (y)
_____ ratio from
________________
Converting grams to grams
Many stoichiometry problems follow a pattern:
grams(x)  moles(x)  moles(y)  grams(y)
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 and
mole ratio in the balanced equation.
Have we learned it yet?
Try these on your own -
4 NH3 + 5 O2  6 H2O + 4 NO
a) How many moles of H2O can be made using 1.6 mol NH3?
b) what mass of NH3 is needed to make 0.75 mol NO?
c) how many grams of NO can be made from 47 g of NH3?