Stoichiometry:
Quantitative Information
About Chemical Reactions
Chapter 4
4.1 Mass Relationships
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stoichiometric factor: a mole ratio
based on the coefficients of reactants
or products in the balanced equation
What mass of oxygen, O2, is required
to completely combust 454 g of
propane, C3H8? What masses of CO2
and H2O are produced?
C3H8 + 5O2  3CO2 + 4H2O
4.2 Limiting Reactants
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A limiting reactant is determines the
amount of product formed.
Step 1: Find the amount of each
reactant.
Step 2: What is the limiting reactant?
Step 3: Calculate the mass of product.
Step 4: Calculate the mass of excess
reactant.
Limited Reactants
Pure silicon, required for computer
chips and solar cells, is made by the
reaction
SiCl4 + 2Mg  Si + 2MgCl2
If you begin with 225 g each of SiCl4
and Mg, which is the limiting reactant
in this reaction? What quantity of Si,
in grams, can be produced?
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4.3 Percent Yield
% yield = actual yield x 100
theoretical yield
Methanol, CH3OH, can be burned in oxygen to provide
energy, or it can be decomposed to form hydrogen
gas, which can then be used as a fuel.
CH3OH  2H2 + CO
If 125 g of methanol is decomposed, what is the
theoretical yield of hydrogen? If only 13.6 g of
hydrogen is obtained, what is the percent yield of
the gas?
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4.4 Quantitative Analysis
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A substance of unknown concentration
may be allowed to react with a known
quantity of something else
OR
An unknown substance can be
converted to one or more substances
of known composition (back track)
Chemical Analysis
One method for determining the purity of a
sample of titanium (IV) oxide, TiO2, an
important industrial chemical, is to combine
the sample with bromine trifluoride.
3TiO2 + 4BrF3  3TiF4 + 2Br2 + 3O2
Oxygen gas is evolved quantitatively. The gas
can be captured readily, and its mass can
be determined. Suppose 2.367 g of a TiO2containing sample evolves 0.143 g of O2.
What is the mass percent of TiO2 in the
sample?
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Determining an Unknown
Formula
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The unknown pure compound is
decomposed into known products.
The reaction products are isolated in pure
form and the amount of each is determined.
The amount of each product is related to
the amount of each element in the original
compound to give the empirical formula.
Determining an Empirical
Formula
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A 0.523 g sample of the unknown
compound CxHy is burned in air to give
1.612 g of CO2 and 0.7425 g of H2O.
A separate experiment gave a molar
mass for CxHy of 114 g/mol.
Determine the empirical and molecular
formulas for the hydrocarbon.
4.5 Concentrations of
Compounds in Solution
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1 mole element = 6.02 x 1023 atoms
1 mole molecular compound = 6.02 x
1023 molecules
1 mole ionic compound = 6.02 x 1023
formula units
Molarity
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Molarity = moles of solute per liter of
solution
Example: Prepare 2.00L of 0.250M
NaOH from solid NaOH.
– Place 20.0g NaOH in a 2L volumetric
flask; add water to dissolve the NaOH,
and fill to the mark with water, mixing
several times along the way.
Molarity
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Sodium bicarbonate, NaHCO3, is used
in baking powder formulations and in
the manufacture of plastics and
ceramics, among other things. If 26.3
g of the compound is dissolved in
enough water to make exactly 200mL
of solution, what is the molar
concentration of NaHCO3? What are
the concentrations of the ions?
Preparations of Known
Solutions
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An experiment in your laboratory
requires 250.mL of a 0.0200M solution
of AgNO3. You are given solid AgNO3,
distilled water, and a 250.mL
volumetric flask. Describe how to
make up the required solution
Dilutions
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M1V1 = M2V2
Prepare 2.00L of 0.250M NaOH from
1.00M NaOH.
Add 500.mL of 1.00M NaOH stock
solution to a 2L volumetric flask; add
water, mixing until the flask is filled to
the mark on the neck of the flask.
Preparing a Solution by
Dilution
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In one of your laboratory experiments,
you are given a solution of CuSO4 that
has a concentration of 0.15M. If you
mix 6.0mL of this solution with enough
water to have a total volume of
10.0mL, what is the concentration of
CuSO4 in this new solution?
Preparing a Solution by
Dilution
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An experiment calls for you to use
250. mL of 1.00M NaOH, but you are
given a large bottle of 2.00M NaOH.
Describe how to make the 1.00M
NaOH in the desired volume.
4.6 pH, a Concentration
Scale for Acids and Bases
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pH can be calculated directly from
hydrogen ion concentration
pH = -log[H+]
Can use the antilog of pH to figure out
hydrogen ion concentration
[H+] = 10-pH
pH
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What is the pH of a solution of HCl,
where [HCl] = 2.6 x 10-2 M?
What is the hydrogen ion
concentration in saturated calcium
hydroxide with a pH of 12.45?
4.7 Stoichiometry of
Reactions in Solution
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If you combine 75.0 mL of 0.350M HCl
and an excess of Na2CO3, what mass
of CO2 (in grams) should be produced?
Titrations
A titration can be used to
determine the concentration of
an unknown solution. A
substance, present in unknown
quantity, can be allowed to react
with a known quantity of another
substance. If the stoichiometric
ratio for their reaction is known,
the unknown quantity can be
determined.
Titrations
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Acid-base indicators are used to tell
when the reaction is complete.
When the solution changes color, the
equivalence point (also called end
point) has been reached. The amount
of OH- equals the amount of H+.
Amount of base (mol) = conc. of base (M) x volume of
base (L)
Acid-Base Titration
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A 25.0 mL sample of vinegar requires 28.33
mL of a 0.953 M solution of NaOH for
titration to the equivalence point. What
mass (in grams) of acetic acid is in the
vinegar sample, and what is the
concentration of acetic acid in the vinegar?
CH3CO2H + NaOH  NaCH3CO2 + H2O
Titrations
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Often the concentration of the base is
unknown and must be determined
through standardization.
– primary standard: a sample of solid acid
or base is titrated with a solution of the
base or acid to be standardized
– titrate with another solution that is
already standardized
Standardization
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Hydrochloric acid can be purchased
from chemical supply houses with a
concentration of 0.100M, and such a
solution can be used to standardize
the solution of a base. If titrating
25.00 mL of a sodium hydroxide
solution to the equivalence point
requires 29.67mL of 0.100M mL, what
is the concentration of the base?
Redox Titrations
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A sample containing vitamin C is
titrated with Br2, an oxidizing agent.
C6H8O6 + Br2  2HBr + C6H6O6
A 1.00g “chewable” vitamin C tablet
requires 27.85mL of 0.102M Br2 for
titration to the equivalence point.
What is the mass of vitamin C in the
tablet?
4.8 Spectrophotometry
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It is possible to measure the extent of
light absorption through a solution
that is colored, and then you can
relate this to the concentration of the
dissolved solute. This method of
quantitatively measuring light
absorption is called
spectrophotometry.
Transmittance and
Absorbance
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Transmittance is the ratio of the
amount of light transmitted by or
passing through the sample relative to
the amount of light that initially fell on
the sample (the incident light).
Absorbance is defined as the negative
log of the transmittance. As
absorbance increases, transmittance
decreases. Absorbance increases as
concentration increases.
The Beer-Lambert law
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Absorbance increases as path length
increases.
The amount of light absorbed by a
solution can be used to determine the
concentration of the absorbing
molecule in that solution.
A=εxlxc
where ε is the molar absorptivity
Homework for Chapter 4
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After reading chapter 4, you should be
able to do the following…
P. 201 (77-86)