StoiCHIOmetry!
Lesson 1
What is it?
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H3PO4 + KOH → KH2PO4 + H2O
H3PO4 + 2KOH → K2HPO4 + 2H2O
H3PO4 + 3KOH → K3PO4 + 3H2O
Each product is useful in their own right
We can change what product we get
depending on how much KOH we add with
relative to H3PO4
Stoichiometry
• The relationship between the amount of reactant
used in a chemical reaction and the amount of
product produced by a BALANCED reaction
• Think of baking
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2 cups of flour
1 egg
3 table spoons of sugar
1 table spoon of baking soda
1 cup of milk
Stoichiometry
• Reactants and products exist in
PROPORTIONAL RELATIONSHIPS!
• It is just a fancy word saying the mole concept
between reactants and products.
• So we can calculate how much product we
produce if we know how much reactant we
started with.
Lets take a look!
• Coefficients tells us how much of each we
must have
• 2H2 + O2 → 2H2O
• We have 2 hydrogen gas molecules react with
1 oxygen gas molecule to produce two water
molecules
• HOWEVER, we can write it like this
• 200H2 + 100O2 → 200H2O
Stoichiometry
• 1.20 x 1024H2 + 6.02 x 1023O2 → 1.20 x 1024H2O
• These are all still balanced
• Remember Avagadros number is the amount of
particle/molecules/atoms per mole.
• 6.02 x 1023 particles = 1 mole
• 2 mol H2 + 1 mol O2 → 2 mol H2O
• 2H2 + O2 → 2H2O
• So now our coefficients is the number of moles!
Molar Ratio
• It is our proportional relationship
• A ratio formed between the mole coefficient
of the REQUIRED (what we are looking for)
chemical substance to the mole coefficient of
the GIVEN chemical substance
• Mole coefficient of required substance
---------------------------------------------------------• Mole coefficient of given substance
What we want is always on top!
• So if we were looking at the water reaction.
• We want to know how much water we created
and our O2 moles are given, we can put it into a
mole ratio
• 2H2 + O2 → 2H2O
• 2 mol H2O / 1 mol O2
• Required chemical amount = what the question is
looking for
• Given chemical amount = what information did
the question already give.
Example
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4NH3 + 7O2 → 4NO2 + 6H2O
What is the mole ratio of NH3 to O2?
NH3 and NO2?
NH3 and H2O?
These 3 cases all follow the CONSERVATION
OF MATTER/ATOM
– Atoms can neither be produced or destroyed!
Using the molar ratio
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We can do a lot of things
Calculate the amount of products formed
Calculate the amount of reactants used up
The molar ratio is always the same with a
BALANCED reaction.
– Hence why balancing is so important
Example
• N2 + 3H2 → 2NH3
• The mole ratio is 1 mol N2 to 3 mol H2 to 2 mol
of NH3
• If we have 2.00 mol of N2 reacting with H2,
how many moles of NH3 will be produced?
• NOTE
– It is important to WRITE the CHEMICAL FORMULA
beside the mole now as we are working with more
than 1 type of mole.
Example
• N2 + 3H2 → 2NH3
• We have 6.00 mole of H2 reacted with N2, how
many moles of NH3 will be produced?
Molar Ratio / Mole Ratio / ?
• So we call that molar ratio a special name
• We call it a mole bridge because it BRIDGES
the reactants with the products.
• Page 125 of your Hebden has the same
diagram!
Steps to solving questions!
• To solve any question, we follow these steps
1. Predict the chemical reaction
2. Balance the chemical reaction
3. If given an amount in mass/volumes/particles,
convert it to moles
4. Do the mole ratio by crossing the mole bridge
5. Convert the amount of moles you found to
mass/volumes/particles.
• KEY HERE IS TO CONVERT TO MOLES FIRST!
Example
• 2KClO3 → 2KCl + 3O2
• If 1.50 mol of KClO3 decomposes. How many
grams of O2 will be produced?
• So we already have the moles of KClO3, just
cross the mole bridge!
• Remember, what we want is ALWAYS ON TOP
for the mole bridge!
Example
• 2KClO3 → 2KCl + 3O2
• If 121.0g of O2 is produced, how many moles
of KClO3 were decomposed?
• We want to produce 2.75 mol of KCl. How
many grams of KClO3 would be required?
– You try!
Adding another step (mass to mass)
• If we go from mass to mass, we must convert
our first mass to moles.
• How many grams of chlorine gas can be
liberated from the decomposition of 64.0g of
AuCl3?
– 2AuCl3 → 2Au + 3Cl2
Example
• Calculate the mass of AgCl that can be
prepared from 200.0g of AlCl3 and sufficient
AgNO3
• 3AgNO3 + AlCl3 → 3AgCl + Al(NO3)3
Example
• How many grams of Na are required to react
completely with 75.0g of chlorine gas?
– 2Na + Cl → 2NaC
Homework
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Page 124 #1-5
Page 127 #6-16 (even only)
Exit Slip worksheet
Mole Ratio Worksheet
Molar Ratios Practice Worksheet
Using the Mole Bridge Worksheet