Stoichiometry
Chapter 12
12. 1 What is stoichiometry?
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
Stoichiometry is the study of quantitative
relationships between the amounts of
reactants used and products formed by a
chemical reaction.
It is based on the law of conservation of
mass, which says that matter is not created
or destroyed.
Balanced Chemical Equations

The coefficients in a balanced equation can
tell you two things:
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
How many individual particles are interacting
in the chemical reaction.
How many moles of particles interact to form
the reaction.
The coefficients do not tell you directly the
mass of reactants and products in the
reaction.
Problems
1.
Interpret each balanced equation in terms
of particles, moles, and mass. Show that
the law of conservation of mass is
observed.
a.
b.
c.
2H2O2 (l) O2 (g) + 2H2O (l)
H2CO3 (aq)  H2O (l) + CO2 (g)
4HCl (aq) + O2(g)  2H2O (l) + 2Cl2 (g)
Mole Ratio


Since coefficients show relationships in the
moles of reactants and products, they can be
used to write mole ratios.
A mole ratio is a ratio between the number
of moles of any two substances in a
balanced chemical equation.
Problems
2.
Determine all the mole ratios for the
following balanced chemical equations.
a.
b.
c.
N2 (g) + O2 (g)  2NO (g)
4NH3 (aq) + 5O2 (g)  4NO (g) +6H2O (l)
4HCl (aq) + O2 (g)  2H2O (l) + 2Cl2 (g)
Stiochiometric Calculations
Section 12.2
Stoichiometric Calculations

Three basic stoichiometric calculations:
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

Mole-to-mole conversions
Mole-to-mass conversions
Mass-to-mass conversions
Steps for Calculations
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

The first thing you need, before you can do
any stoichiometric calculations, is to have a
balanced chemical equation.
Remember, when you are converting from
one substance to another you must compare
the quantities using the units moles.
Moles are the only basis of comparison
between two different substances.
Mole-to-Mole Conversions
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
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
How can you determine the number of
moles of table salt (NaCl) produced from
0.02 moles of chlorine (Cl2)?
First, begin with the balanced chemical
equation.
2Na (s) + Cl2 (g)  2NaCl (s)
Then use the mole ratio to convert the
known moles of chlorine to the number of
moles of table salt.
Example
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A piece of magnesium burns in the presence of
oxygen, forming magnesium oxide (MgO). How
many moles of oxygen are needed to produce
12 moles of magnesium oxide?
First, write the equation.
2Mg (s) + O2 (g) 2MgO (s)
1 mole oxygen = 2 molmagnesium oxide
Set up a dimensional analysis problem to solve.
Problem
3.
The carbon dioxide exhaled by astronauts
can be removed from a spacecraft by
reacting it with lithium hydroxide (LiOH).
The reaction is as follows: CO2 (g) + 2LiOH
(s) Li2CO3 (s) + H2O (l). An average
person exhales about 20 moles of CO2 per
day. How many moles of LiOH would be
required to maintain two astronauts in a
spacecraft for three days?
Problem
Balance the following equation and answer
the question below.
KClO3 (s)  KCl (s) + O2 (g)
4.
a.
b.
c.
How many moles of O2 are produced from 10
moles of KClO3?
How many moles of KCl are produced using 3
moles of KClO3?
How many moles of KClO3 are needed to
produce 50 moles of O2?
Mole-to-Mass conversion


Remember, all mass to mole conversions are
completed by using the molar mass (the
mass of 1 mole of the substance).
Set up a dimensional analysis problem to
solve.
Problem
Calculate the mass of sodium chloride
(NaCl) produced when 5.50 moles of
sodium react in excess chlorine gas.
How many grams of chlorine gas must be
reacted with excess sodium iodide (NaI) to
produce 6.00 moles of sodium chloride?
e.
f.
a.
a.
Balance the equation:
NaI (aq) + Cl2 (g)  NaCl (aq) + I2 (s)
Perform the calculation
Mass-to-Mass Conversions
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In order to compare two different quantities
remember that you must use the mole ratio.
So, first convert the mass of the first
substance to moles.
Then convert the moles of the first
substance to moles of the second substance.
Then convert the moles of the second to
mass of the second substance.
Example

How many grams of sodium hydroxide
(NaOH) are needed to completely react with
50.0 grams of sulfuric acid (H2SO4) to form
sodium sulfate (Na2SO4) and water?
Problem
Balance each equation and solve the problem.
8.
a.
b.
c.
If 40.0 g of magnesium reacts with excess hydrochloric
acid (HCl), how many grams of magnesium chloride
(MgCl2) are produced?
Mg (s) + HCl (aq)  MgCl2 (aq) + H2 (g)
Determine the mass of copper needed to react
completely with a solution containing 12.0 g of silver
nitrate (AgNO3)?
Cu (s) + AgNO3 (aq)  Cu(NO3)2 (aq) + Ag (s)
How many grams of hydrogen chloride (HCl) are
produced when 15.0 g of sodium chloride (NaCl) reacts
with excess sulfuric acid (H2SO4)?
NaCl (aq) + H2SO4 (aq) Na2SO4 (s) + HCl (g)
Section 12.3
LIMITING REACTANTS
Limiting Reactants
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Rarely are the reactants in a chemical
reaction present in the exact mole ratio
shown in the balanced equation.
When one reactant is used completely up
first, it is called the limiting reactant.
The limiting reactant limits the reaction and
thus determines how much product forms.
The left-over reactants are called the excess
reactants.
Limiting Reactant
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To find out which reactant is the limiting
reactant, convert each of the reactants to
moles of product.
The reactant that produces the smallest
amount of product is the limiting reactant.
The other reactants are excess reactants.
Excess Reactant
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Once the limiting reactant has been found,
you can find the amount of excess that is
left-over after the reaction is finished.
To do this, convert the limiting reactant to
mass of the excess reactant.
Then subtract the mass of the excess from
the initial amount of the excess reactant.
Example

In the reaction below, 40.0 g of sodium
hydroxide (NaOH) reacts with 60.0 g of
sulfuric acid (H2SO4).
2NaOH (aq) + H2SO4 (aq)  Na2SO4 (aq) + 2H2O (g)
a.
b.
Which reactant is the limiting reactant?
What mass of Na2SO4 can be produced using the given
quantities of the reactants?
Movie
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Limiting Reactants
Problems
Ammonia (NH3) is one of the most common
chemicals produced in the United States. It is used
to make fertilizer and other products. Ammonia is
produced by the following chemical reaction.
N2 (g) + 3H2 (g)  2NH3 (g)
9.
a.
b.
c.
If you have 1.00 X 103 g of N2 and 2.50 X 103 g of H2,
which is the limiting reactant in the reaction?
How many grams of ammonia can be produced from
the amount of limiting reactant available?
Calculate the mass of excess reactant that remains
after the reaction is complete.
Problem
10.
a.
a.
b.
c.
Aluminum reacts with chlorine to produce
aluminum chloride.
Balance the equation:
Al (s) + Cl2 (g)  AlCl3 (s)
If you begin with 3.2 g of aluminum and 5,4 g of
chlorine, which is the limiting reactant?
How many grams of aluminum chloride can be
produced from the amount of limiting reactant
available?
Calculate the mass of excess reactant the remains
after the reaction is complete.
Section 12.4
PERCENT YIELD
Yield
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Most reactions do not produce the predicted
amount of product.
Many reactions stop before all the reactants
are used up, so less product is formed than
expected.
Also, sometimes products other than those
expected are formed, and so they reduce the
amount of the desired product.
Theoretical Yield
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
The theoretical yield is the maximum
amount of product that can be produced
from a given amount of reactant under ideal
circumstances.
It is the amount that you calculate using
stoichiometry.
Actual Yield
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
The actual yield is the amount of product
that is actually produced in an experiment.
It is what you get when you do the lab.
Percent Yield

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The percent yield is the ratio of the actual
yield to the theoretical yield expressed as a
percent.
Percent yield = Actual yield * 100%
Theoretical yield
The percent yield tells you how efficient a
chemical reaction is in producing the desired
product.
Example

Aspirin (C9H8O4) can be made from salicylic
acid (C4H6O3). Suppose you mix 13.2 g of
salicylic acid with an excess of acetic
anhydride and obtain 5.9 g of aspirin and
some water. Calculate the % yield of
aspirin.
2C7H6O3 (s) + C4H6O3 (l) 2C9H8O4 (s) + H2O (l)
Problem
11. Calculate the percent yield for each
chemical reaction based on the data
provided.
a.
Theo. Yield: 25 g; act. Yield: 20 g
b.
Theo. Yield: 55g; act. Yield: 42g
c.
Theo. Yield: 5.2 g; act. Yield: 4.9 g
Problem
12.
In an experiment, 10.0 g of magnesium
reacted with excess hydrochloric acid
forming magnesium chloride.
Mg (s) + 2HCl (aq)  MgCl2 (aq) + H2 (g)
At the completion of the reaction, 29.5 g of
magnesium chloride was produced.
Calculate the theoretical yield and the
percent yield.