Stoichiometry

1
Chapter 3 Stoichiometry
2
Law of Conservation of Mass
Lavoisier The total mass of all substances
present after a chemical reaction (the products)
is the same as the total mass of all substances
present before the reaction (the reactants)
1787 Irenee du Pont was apprenticed to
Lavoisier. Later founded the company as a
provider of gunpowder.
1794 Lavoisier is executed just months before
the execution of Robespierre, which ended the
Reign of Terror in France.
3
Stoichiometry
Stoichiometry includes all the quantitative
relationships involving atomic and formula
masses chemical formulas amount (moles) of
substances
4
Determining Percent Composition From a Chemical
Formula
The formula mass (also called molecular mass,
formula weight or molecular weight) is the total
mass of all the elements that make up the compound
5
Aspartame
6
How many atoms in 1 g of copper?
If Cu has an atomic mass of 63.546 amu, how does
this easily convert to grams?
Avagadros Number (the mole) relates atomic mass
to sample mass
NA 6.022 X 1023 1 mole
1 mole of atoms mass in grams equal to the
atomic mass
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How many atoms in 1 g of copper?
Therefore
If one atom of Cu has a mass of 63.546 amu,
then 1 mole of Cu has a mass of 63.546 g and
63.546 g of Cu contains 6.022 X 1023 atoms of Cu
NA 6.022 X 1023 1 mole
1 mole of atoms mass in grams equal to the
atomic mass
The molar mass, M, is the mass of one mole of a
substance.
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An unknown compound is 26.1 carbon, 4.4
hydrogen and 69.5 oxygen by mass. What is the
empirical formula?
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Finding an Empirical Formula From Mass Percent

• Choose an arbitrary sample size (100g is a good
  choice)
• Convert masses to moles
• Write a formula with the molar values as
  subscripts
• Divide each subscript by the smallest value
• Multiply all subscripts by a small whole number
  to make the subscripts integral (if necessary)

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Representing Chemical Reactions
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Balancing Chemical Reactions

• 1st Balance elements that occur in only one
  compound on each side
• Balance free elements last
• (Balance unchanged polyatomics as groups)
• Clear fractional coefficients by multiplication
• Never introduce extraneous atoms
• Never change chemical formulas

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Equations and the Mole Concept
13
What mass of water is produced when 42.0 ml
ethanol is burned completely? (density of ethanol
0.790 g/ml)
14
You are a biologist and you are studying the
mating habits of the Asian Civet cat.
You manage to isolate 62 mg of the sex
pheromone Combustion analysis of the pheromone
produces 138.0 mg CO2 and 49.4 mg H2O
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Combustion Analysis
Image from cwx.prenhall.com/petrucci
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The Sex Pheromone For The Asian Civet Cat
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Limiting Reagents
We are making bagel pizzas (pepperoni, of
course). For each pizza, we need 1 bagel, 1 oz.
tomato sauce, 2 slices of cheese and 5 slices of
pepperoni
If we have 13 bagels, 20 oz. of tomato sauce, 26
slices of cheese and 60 slices of pepperoni, how
many pizzas can we make?
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If we react 85.0 g of Zn metal with 35.2 g of
HCl, what is the mass of H2 gas that is produced?
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Actual Theoretical Yield
Theoretical Yield The maximum amount of product
that can be produced (usually reported in mol
or g) Actual Yield Real (measured) amount of
product that was produced/obtained (usually
reported in mol or g) Percent Yield The ratio
of actual yield to theoretical yield (reported
as a percent, ALWAYS 100 or less)
Actual Yield
Percent Yield
X 100
Theoretical Yield
20
We reacted 12.8 g of Al metal with excess HBr and
obtained 77.9 g of AlBr3. What is our percent
yield?