Unit 8 - Stoichiometry
The Math of Chemical Reactions
Recall…
 In our last unit, we talked about how to
convert between moles & mass and moles
& atoms/molecules for one compound.
 In this unit, we will be applying these skills
to convert between moles and mass of
two different compounds.
What is Stoichiometry?

The study of the mathematical relationships
between the substances in balanced
chemical reactions

Allows for calculation of amounts of
reactants or products in a chemical reaction

i.e. How much of one thing can you make if
you start with a certain amount of another
thing?
Stoichiometry in Chemistry
2 H2 + O 2  2 H2O

Coefficients in a balanced chemical
equation indicate the relative amounts of
each substance involved in the reaction

One way to describe the ratios involved in the
reaction above would be “two molecules of
hydrogen gas plus one molecule of oxygen gas
yields two molecules of water”
Mole Ratios
However, since these are only ratios, using the
same equation,
2 H 2 + O 2  2 H 2O
It is equally true that, “two moles of hydrogen
plus one mole of oxygen yields two moles of
water”

These mole relationships are called
“mole ratios”
Mole Ratio

Mole ratio – conversion factor which relates
moles of one compound to moles of another
compound in a balanced equation

Coefficients from balanced equation are used to
write mole ratios
N2 (g) + 3H2 (g)  2NH3 (g)
1 mol N2
3 mol H2
2 mol NH3
1 mol N2
3 mol H2
2 mol NH3
Mole Ratios
Example #1: What is the mole to mole ratio of
oxygen to water in the following equation:
2 H 2 + O 2  2 H 2O
Mole Ratios
Example #2: Balance the equation and then
determine the mole ratio of sodium hydroxide to
sodium sulfate in the following equation:
___H2SO4 + ___ NaOH  ___Na2SO4 + ___H2O
Warm Up:
1. Balance the equation:
___Na + ___Cl2  ___NaCl
2. What is the molar ratio between Na and
Cl2?
3. What is the molar ratio between Cl2 and
NaCl?
Stoichiometric Calculations
Mole to Mole Calculations – converting from moles of
one compound to moles of a different compound
1.
2.
3.
Write and balance the equation if not already done
Write the mole ratio between the two mentioned
compounds using the coefficients from the
balanced equation. This will serve as your
conversion factor.
Use dimensional analysis & the mole ratio to
convert from moles of compound #1 to moles of
compound #2
Mole to Mole Calculations
Ex #1: In the equation below, if 0.5 moles of
magnesium hydroxide react, how many moles of
water would be produced?
2 H3PO4 + 3 Mg(OH)2  Mg3(PO4)2 + 6 H2O
Mole to Mole Calculations
Ex #2: In the equation below, if 0.246 moles of
H3PO4 react, how many moles of water would be
produced?
2 H3PO4 + 3 Mg(OH)2  Mg3(PO4)2 + 6 H2O
Mole Conversions WS – Prob. 1
Aluminum metal and hydrogen chloride react to form aluminum
chloride and hydrogen gas.
How many moles of aluminum metal are needed to produce 3.33
moles of aluminum chloride?
Mole Conversions WS - Answers
1. a. 3.33 mol Al
b. 1.64 mol HCl
2. a. 4.65 mol NaBr
b. 1.55 mol Al(OH)3
3. a. 3.5 x 10-4 mol CH4 b. 7.0 x 10-4 mol O2
Warm-Up:
Balance the following equation:
__N2 + __H2  __NH3
If you have 4.5 moles of hydrogen gas, how
many moles of ammonia (NH3) would you
produce?
General Tips for Stoic. Problems

Always convert the given value to moles as
your first step!

Once you get to moles, you can convert to
moles of the unknown
Moles of unknown can then be converted to
whatever final units the problem asks for
(grams, liters, milliliters, molecules, etc.)

Mass to Mass Calculations
1.
2.
3.
Convert from grams of given compound to moles of
given compound (using molar mass)
Convert from moles of given to moles of unknown
(using the mole ratio)
Convert from moles of unknown to grams of
unknown (using molar mass)
Mass to Mass Calculations
Master Formula for Mass to Mass Calculations
Given mass (g)

1 mol of given
Coefficient of unknown (mol) Molar mass unknown (g)


Molar mass given (g)
Coefficient of given (mol)
1 mol of unknown
Mass to Mass Calculations
Ex #1: In the unbalanced equation below, how
many grams of carbon dioxide will be produced
by the reaction of 108 grams of C5H12?
___ C5H12 + ___ O2  ___ CO2 + ___ H2O
Mass to Mass Calculations
Ex #2: In the balanced equation below, how
many grams of C5H12 would be needed to
completely react with 11.5 grams of oxygen?
C5H12 + 8 O2  5 CO2 + 6 H2O
Answers for Mass-Mass WS
1.
2.
3.
15.2 g KCl
A. 10.7 g H2
A. 4.2 g AgCl
B. 60.7 g NH3
B. 4.6 g BaCl2
Percent Yield

Theoretical yield – amount of product that should
be formed when the limiting reactant (the
reactant that runs out first) is completely used up

Actual yield – the actual amount of product
produced when the reaction is carried out
experimentally

actual yield
Percent yield =
 100%
theoretical yield
Percent Yield
actual yield
 100%
theoretical yield
Example #1: If a student made 1.72 g of NaCl
in the lab when they should have made 2.00 g,
what is the percent yield?
Percent Yield
actual yield
 100%
theoretical yield
Example #2:
N2 + 3 H2  2 NH3
A student calculates that for the reaction above,
5.0 grams of NH3 should be produced. After
carrying out the reaction, the student made 4.5 g
of NH3. What is the percent yield of the reaction?
Example #3
___ZnCl2 + ___(NH4)3PO4  ___Zn3 (PO4)2 + ___NH4Cl
(a) How many grams of zinc phosphate will be produced
by the reaction of 5.00g of ammonium phosphate?
(b) If a student did this reaction and produced 2.13 g of
zinc phosphate, what is their % yield?
Example #3
___ZnCl2 + ___(NH4)3PO4  ___Zn3 (PO4)2 + ___NH4Cl
(a) How many grams of zinc phosphate will be produced
by the reaction of 5.00g of ammonium phosphate?
Example #3
___ZnCl2 + ___(NH4)3PO4  ___Zn3 (PO4)2 + ___NH4Cl
(a) How many grams of zinc phosphate will be produced
by the reaction of 5.00g of ammonium phosphate?
(b) If a student did this reaction and produced 2.13 g of
ammonium phosphate, what is their % yield?
Answers to Percent Yield WS
1. a. 33.1 g
b. 56%
2. a. 8.2 g
b. 76%
3. 42%
Answers to HW #5
0.290 mole H2O
2. 1.68 g H2O
3. a. 1.62 g H3PO4
b. 93% (Good)
1.
Limiting Reactant

The reactant that is used up first in a
chemical reaction
 Determines how much product can be
formed by a reaction
 The reactant with fewer moles present at
the beginning of the reaction
Excess Reactant

Reactant present in a quantity that is more
than sufficient to react with the limiting
reactant
 Any reactant that remains after all the
limiting reactant is used up
 The reactant present with more moles at
the beginning of the reaction
Limiting Reactant Balloon Demo
What was the limiting reactant for the first balloon?
_____________________________
MMbaking soda= 84.01 g/mol NaHCO3
MMvinegar= 60.05 g/mol CH3COOH
Moles baking soda:
Moles vinegar:
Limiting Reactant Balloon Demo
What was the limiting reactant for the second balloon?
_____________________________
MMbaking soda= 84.01 g/mol NaHCO3
MMvinegar= 60.05 g/mol CH3COOH
Moles baking soda:
Moles vinegar:
Balloon Demo Questions

Why were the third and fourth balloons the
same size even though more baking soda
was added to the fourth balloon?

Limiting reactant 3rd balloon:

Limiting reactant 4th balloon: