STOICHIOMETRY

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USING THE CHEMICAL
EQUATION LIKE A RECIPE
STOICHIOMETRY
Stoichiometry
 What is It?
 Quantitative relationships
 In chemical reactions
 Based on law of Conservation of
Mass
 Why/How is it Useful?
Straight from Wikipedia
 Stoichiometry (pron.: /ˌstɔɪkiˈɒmɨtri/) is a
branch of chemistry that deals with the relative
quantities of reactants and products in chemical
reactions. In a balanced chemical reaction, the
relations among quantities of reactants and
products typically form a ratio of positive
integers. For example, in a reaction that forms
ammonia (NH3), exactly one molecule of
nitrogen (N2) reacts with three molecules of
hydrogen (H2) to produce two molecules of NH3:
 N2 + 3H2 → 2NH3
Stoichiometry is a lot like
making tacos.
Taco Recipe
What if I need to make tacos
for 50 people?
 1o shells
 You know how much
 1 lb ground beef
product you need (tacos)
 You have to calculate how
much of each ingredient
you need.
 That’s stoichiometry!!!!
 1 pkg of taco seasoning
 ¾ c water
 ½ cup salsa
 1 cup cheddar cheese
 1 cup of lettuce
Makes 10 tacos.
 Equations are a chemist’s recipe.
 Equations tell chemists
 what amounts of reactants to mix and
what amounts of products to expect.
 If you know the quantity of one
substance in a reaction
 you can calculate the quantity of any other
substance consumed or created in the
reaction
 Quantity meaning the amount of a
substance in grams, liters, molecules,
or moles.
Balanced Chemical Equations
 The coefficients in a balanced chemical
equation shows how many moles/particles of
one reactant are needed to react with other
reactants and how many moles/particles of
product will be formed.
 Based on Law of Conservation of Mass
4Fe + 3O2  2Fe2O3
4Fe + 3O2  2Fe2O3
 Write the relationships we can derive from
this equation.
C3H8 + 5O2  3CO2
+ 4H2O
 The combustion of propane provides energy
for heating homes, cooking food, and
soldering metal parts. Interpret the equation
for the combustion of propane in terms of
particles, moles, and mass.
 Show that the law of conservation of mass
is observed
C3H8 + 5O2  3CO2
+ 4H2O
 The calculation of quantities in chemical
reactions is called stoichiometry.
 Imagine you are in charge of manufacturing for Rugged Rider Bicycle
Company.
 The business plan for Rugged Rider
requires the production of 128 custommade bikes each day.
 You are responsible for insuring that
there are enough parts at the start
of each day.
An Analogy for Stoichiometry
 Assume that the major components
of the bike are the frame (F), the seat
(S), the wheels (W), the handlebars
(H), and the pedals (P).
 The finished bike has a “formula” of
FSW2HP2.
 The balanced equation for the
production of 1 bike is.
F +S+2W+H+2P  FSW2HP2
 Now in a 5 day workweek, Rugged Riders
is scheduled to make 640 bikes. How
many wheels should be in the plant on
Monday morning to make these bikes?
 What do we know?
 Number of bikes = 640 bikes
 1 FSW2HP2=2W (balanced eqn)
 What is unknown?
 # of wheels = ? wheels
 The connection between wheels and
bikes is 2 wheels per bike. We can use
this information as a conversion factor
to do the calculation.
640 FSW2HP2
2W
1 FSW2HP2
= 1280
Wheels
• We can make the same kinds of
connections from a chemical rxn eqn.
N2(g) + 3H2(g)  2NH3(g)
• The key is the “coefficient ratio”.
 The coefficients of the balanced chemical
equation indicate the numbers of moles
of reactants and products in a chemical
reaction.
 1 mole of N2 reacts with 3 moles of H2 to
produce 2 moles of NH3.
 N2 and H2 will always react to form
ammonia in this 1:3:2 ratio of moles.
 So if you started with 10 moles of N2 it
would take 30 moles of H2 and would
produce 20 moles of NH3
 Using the coefficients, from the balanced
equation as ratios to make connections
between reactants and products, is the
most important information that an
equation provides.
 Using this information, you can calculate
the amounts of the reactants involved
and the amount of product you might
expect.
Mole Ratios
 Shows the mole-to-mole ratio between two
of the substances in a balanced equation
 Written from the coefficients of any two
substances in the equation
 This is a very important conversion and the
key to solving stoichiometry problems.
2Al + 3Br2
 2AlBr3
4 Fe
+
3 O2 
2 Fe2O3
3 H2(g)
+
N2(g) 2 NH3(g)
A mol ratio for H2 and N2
A mol ratio for NH3 and H2
Stoichiometric Calculations
 How much product can be obtained from a
given reaction?
 How much reactant is needed to produce this
much product?
Review of Mole Conversions

Three mole equalities
23
10
1 mol = 6.02 x
particles
1 mol = (molar mass) (from periodic table)
1 mol = 22.4 L for a gas at STP
Review practice
 How many moles are in 5.0 x 1025
molecules of CO2?
 What is the mass in grams of 0.250 moles
of CO2?
 How many moles of CO2 gas are in 100.0 L
at STP?
Types of Stoichiometry
Calculations
 Mole - Mole Calculations
 Moles of Known  Moles of Unknown
 Mole-Mass Calculations
 Moles of Known  Mass of Unknown
 Mass-Mole Calculations
 Mass of Known  Moles of Unknown
 Mass-Mass Calculations
 Mass of Known  Mass of Unknown
MOLE – MOLE EXAMPLE
 The following reaction shows the
synthesis of aluminum oxide.
3O2(g) + 4Al(s)  2Al2O3(s)
• If you only had 1.8 mols of Al how much
product could you make?
 Solve for the unknown:
3O2(g) + 4Al(s)  2Al2O3(s)
1.8 mol Al
2 mol Al2O3
4 mol Al
Mole Ratio
= 0.90mol
Al2O3
 If you wanted to produce 24 moles of
product how many moles of each reactant
would you need?
3O2(g) + 4Al(s)  2Al2O3(s)
 Solve for the unknowns:
3O2(g) + 4Al(s)  2Al2O3(s)
24 mol Al2O3
24 mol Al2O3
4 mol Al
2 mol Al2O3
3 mol O2
2 mol Al2O3
= 48 mol Al
= 36 mol O2
Practice Problem
How many moles of hydrogen will be
produced if 0.44 mol of CaH2 reacts
according to the following equation?
CaH2 + 2H2O  Ca(OH)2 + 2H2
Iron will react with oxygen to produce Iron III
oxide. How many moles of Iron (III) oxide will
be produced if 0.18 mol of Iron reacts?
4
4 Fe
+
3 O2

2
Fe2O3
4 Fe
+
3 O2

2
Fe2O3
How many moles of Fe2O3 are produced
when 6.0 moles O2 react?
How many moles of Fe are needed
to react with 12.0 moles of O2?
31
MASS – MASS CALCULATIONS
 No lab balance measures moles directly,
generally mass is the unit of choice.
 From the mass of 1 reactant or product,
the mass of any other reactant or
product in a given chemical equation can
be calculated, provided you have a
balanced equation.
 As in mole-mole calculations, the
unknown can be either a reactant or a
product.
Example # 1 Mass-Mass
Acetylene gas (C2H2) is produced
by adding water to calcium
carbide (CaC2).
CaC2 + 2H2O  C2H2 + Ca(OH)2
How many grams of C2H2 are produced by
adding water to 5.00 g CaC2?
Step 1: “Get to Moles!” in this case that can
be done by using the Molar Mass of your
given compound.
5.0 g CaC2
1 mol CaC2
64.0 g CaC2
= .07813 mol
CaC2
Step 2: Now we are ready for the KEY
step…converting from mols of our given to
mols of unknown using the mole ratio.
1 mol C2H2
.07813 mol
CaC2 1 mol CaC
2
= .07813 mol
C2H2
Step 3: Since we are asked for mass of our
unknown in this problem, we need to use our molar
mass of our unknown and convert our newly
calculated mols into grams.
26.0 g C2H2
.07813 mol
C2H2 1 mol C H
2 2
= 2.03 g C2H2
Summary of 3 Steps of Stoichiometry
Problems
1. Get to Moles of Given
2. Mole Ratio to calculate moles of unknown
3. Get to wanted final unit
The double replacement reaction between
Lead II nitrate and Potassium Iodide produces
a bright yellow precipitate that can be used as
a color additive in paint. How many grams of
potassium iodide would we need to completely
react 25.3 g of lead (II) nitrate?
Pb(NO3)2 + 2 KI  PbI2 + 2 KNO3
mass A  mols A  mols B  mass B
25.3 g Pb(NO3)2
1mol Pb(NO3)2
331.2g Pb(NO3)2
2mol KI
1mol Pb(NO3)2
166 g KI
1mol KI
= 25.4 g KI
Practice Problems
What mass of Barium chloride is needed to
react completely with 46.8 g of Sodium
phosphate according to the following
equation?
BaCl2 + Na3PO4  Ba3(PO4)2 + NaCl
Another problem
Use the equation to determine what mass of FeS
must react to form 326g of FeCl2.
FeS + 2HCl  H2S + FeCl2
Moles of Known  Mass of Unknown
 Find moles of unknown using the mole ratio.
 Convert moles of unknown to mass of unknown.
 Calculate the mass of carbon dioxide produced
by the decomposition of 2.5 moles of copper(II)
carbonate.
CuCO3  CuO + CO2
Mass to Moles Calculations
 Mass of known  moles of unknown
 Convert mass of known to moles of known.
 Use mole ratio to find moles of unknown.
 Calculate the moles of water produced by
the reaction of 20 grams of hydrogen gas
with excess oxygen gas.
 A balanced reaction equation indicates
the relative numbers of moles of
reactants and products.
 We can expand our stoichiometric
calculations to include any unit of
measure that is related to the mole.
 The given quantity can be expressed in
numbers of particles, units of mass, or
volumes of gases at STP.
 The problems can include mass-volume,
volume-volume, and particle-mass
calculations.
 In any of these problems
1. the given quantity is first converted to
moles.
2. Then the mole ratio from the balanced
equation is used to convert from the
moles of given to the number of moles
of the unknown
3. Then the moles of the unknown are
converted to the units that the
problem requests.
 The next slide summarizes these steps
for all typical stoichiometric problems
Stoichiometry Roadmap
Volume U
Coefficient part U
Coefficient part G
Particles U
1 mole
Molar mass
Balanced Equation
Mass G
Particles G
Moles Unknown
22.4 L
Molar mass
1 mole
22.4 L
1 mole
Volume G
Coefficient mol G
1 mole
Coefficient mol U
Moles Given
Mass U
How many molecules of O2 are
produced when a sample of 29.2 g
of H2O is decomposed by
electrolysis according to this
balanced equation:
2H2O  2H2 + O2
The last step in the production of nitric
acid is the reaction of NO2 with H2O.
How many liters of NO2 must react
with water to produce 5.00x1022
molecules of NO?
3NO2+H2O2HNO3+NO
molecules A mols A mols B volume B
5.0x1022 molecules NO
1 mol NO
3 mol NO2
6.02x1023 molecules NO
1 mol NO
22.4 L NO2
1 mol NO2
= 5.58 L NO2
Aspirin can be made from a chemical reaction
between the reactants salicylic acid and acetic
anhydride. The products of the reaction are
acetylsalicylic acid (aspirin) and acetic acid
(vinegar). Our factory makes 125,000 100count bottles of Bayer Aspirin/day. Each bottle
contains 100 tablets, and each tablet contains
325mg of aspirin. How much in kgs + 10% for
production problems, of each reactant must
we have in order to meet production?
C7H6O3 + C4H6O3  C9H8O4 + HC2H3O2
Salicylic
acid
Acetic
anhydride
aspirin
vinegar
Salicylic Acid:
22,549.4 1 mol C7H6O3
mols
1 mol asp
aspirin
1 kg
1000 g
136.10g C7H6O3
1 mol C7H6O3
= 3068.97 kg salicylic
acid + (306.897 g)
= 3380 kg of salicylic acid
Acetic Anhydride:
22,549.4 1 mol C4H6O3
mols
1 mol asp
aspirin
1 kg
1000 g
102.06g C4H6O3
1 mol C4H6O3
= 2301.39 kg
Acetic anhydride
+ 230.139 kg
= 2530 kg Acetic anhydride
Example
4 Fe + 3 O2
2 Fe2O3
How many grams of O2 are needed to produce 0.400
mol of Fe2O3?
52
Calculating Mass of A
Substance
 Balance equation
 Convert starting amount to moles
 Use coefficients to write a mol-mol ratio
 Convert from moles of known to moles of unknown
 Convert moles of unknown to grams
53
Calculation
The reaction between H2 and O2 produces 13.1 g
of water. How many grams of O2 reacted?
Write the equation
H2 (g) + O2 (g)
H2O (g)
Balance the equation
2 H2 (g) + O2 (g)
2 H2O (g)
54
2 H2 (g) + O2 (g)
?g
2 H2O (g)
13.1 g
55
Points to Remember
1.
2.
3.
4.
Read an equation in moles
Convert given amount to moles
Use mole ratio to find unknown moles
Convert moles to grams
moles (given)
moles (unknown)
grams (given)
grams (unknown)
56
How many O2 molecules will react with 505 grams
of Na to form Na2O?
4 Na + O2
2 Na2O
57
Acetylene gas C2H2 burns in the oxyactylene torch
for welding. How many grams of C2H2 are burned if
the reaction produces 75.0 g of CO2?
2 C2H2 + 5 O2
4 CO2 + 2 H2O
58
Convert moles known to moles of unknown
Use Mole Ratio from Coefficients
Moles of Known
Convert moles to grams
Convert grams to moles
Mass of Known
Moles of Unknown
Mass of Unknow
Limiting Reactants
 The reactant that is not completely used up in
the reaction
 The limiting reactant is not present in sufficient
quantity to react with all other reactants.
 The reaction stops when the limiting reactant is
completely consumed.
 Any remaining reactants are considered "excess
reactants".
 The amount of product formed is determined by the
"limiting reactant"
Steps in solving a limiting
reactant problem
 1. Write a balanced equation for the reaction.
 2. Convert both reactant quantities to moles.
 3. Determine the moles of product that could
be formed by each reactant.
 4. The least amount in step #3 identifies the
limiting reactant.
 5. Use that number of moles of product to
determine the mass produced.
An example of a limiting
reactant problem
 What mass of water can be produced by 4
grams of H2 reacting with 16 grams of O2?
Remember the following
things about limiting
reactants:
 The reaction will stop when the reactants
are used up.
 If one reactant is used up before the other,
the reaction stops then.
 The first reactant used up is the limiting
reactant, use it for the calculation.
 The other reactant is the excess reactant.
 Step 1. Write a balanced equation for the
reaction.
 2 H2 + O2 2 H2O

Step 2. Convert both reactant quantities to moles.
 Step 3. Using the mole ratio from the
equation, determine the moles of water that
could be formed by each reactant.




Step 4. Oxygen produces the least amount of
water.
16 grams of oxygen cannot produce as much
water as 4 grams of hydrogen. In other words,
16 grams of oxygen will be used up in the
reaction before 4 grams of hydrogen.
Oxygen is the "limiting" reactant.
Use oxygen for the calculation of product
amount.
 Step 5.Complete the problem by converting
moles of H2O to mass of H2O.
Analogy
Suppose you are preparing cheese
sandwiches. Each sandwich requires 2
pieces of bread and 1 slice of cheese. If you
have 4 slices of cheese and 10 pieces of
bread, how many cheese sandwiches can
you make?
68
Cheese Sandwich Products
Sandwich 1
+
+
=
Sandwich 2
+
+
=
69
How many sandwiches can you make?
____ slices of bread
+ ____ slices of cheese
= ____ sandwiches
What is left over? ________________
What is the limiting reactant?
70
How many sandwiches can you make?
__10__ slices of bread
+ __4__ slices of cheese
= __4__ sandwiches
What is left over? _2 slices of bread
What is the limiting reactant? cheese
71
Hints for LR Problems
1. For each reactant amount given, calculate the
moles (or grams) of a product it could produce.
2.The reactant that produces the smaller amount of
product is the limiting reactant.
3. The number of moles of product produced by the
limiting reactant is ALL the product possible. There
is no more limiting reactant left.
72
Example
25 grams of hydrogen and 25 grams of oxygen
react to produce water. How much water is
produced?
Percent Yield
 Theoretical yield: Maximum amount of
product that can be obtained from a given
amount of reactant.
 A reaction rarely produces maximum amount.
Things do not go perfectly.
 From the calculation (equation)
 Actual Yield: The amount actually produced.
Percent Yield
 In all of the calculations we have done thus
far the amount of product we have
calculated has been determined in perfect
lab conditions. This means everything goes
exactly right. Everything is ideal.
 This yield of product is called Theoretical
Yield
 Theoretical yield is obtained from
stoichiometric calculations.
 However, in real lab situations, conditions
don’t always work out perfectly.
 When an experiment is carried out it is nice to
be able to determine how efficiently a
reaction has proceeded.
 This can be done by determining the actual
yield produced in the experiment and then
using an equation called percent yield.
 Percent yield is the ratio of the actual
amount of product to the theoretical amount
of product.
Equation
 Percent Yield = Actual Yield/Theoretical Yield
x 100
Example
 You produced 2.5 grams of magnesium
oxide in a reaction of magnesium with
excess oxygen. Calculate the percent yield
for this reaction.
Percent Yield
You prepared cookie dough to make 5 dozen
cookies. The phone rings while a sheet of 12
cookies is baking. You talk too long and the
cookies burn. You throw them out (or give them to
your dog.) The rest of the cookies are okay.
How many cookies could you have made
(theoretical yield)?
How many cookies did you actually make to eat?
(Actual yield)
79
Vocabulary
Actual yield is the amount of product actually
recovered from an experiment
Theoretical (possible) yield is the maximum
amount of product that could be produced from
the reactant. Calculated Yield.
Percent Yield is the actual yield compared to the
maximum (theoretical yield) possible.
80
Percent Yield Calculation
What is the percent yield of cookies?
Percent Yield = Actual Yield (g) recovered X 100
Possible Yield (g)
% cookie yield = 48 cookies x 100 = 80% yield
60 cookies
81

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