STOICHIOMETRY Part II
Mole-Mole Relationships
1
Objectives



Explain how balanced equations apply to both
chemistry and everyday life.
Interpret balanced chemical equations in
terms of moles.
Construct “mole ratios” from balanced
chemical equations, and apply these ratios in
mole-mole stoichiometric calculations.
3
Why Is This Important?

Many things that we use are manufactured
from chemicals.
 Cleaning
products, cosmetics, medications,
food products


For a manufacturer to make a profit, the
cost of making these items must be less
than the money users pay for them.
Chemical processes carried out in industry
must be economical.
 This
is where balanced equations can help
4
Application to Chemistry
• Take the following chemical equation
N2 + 3H2  2NH3
• The key is the “coefficient ratio”
(aka the “mole ratio”)
5
Mole Ratio
The mole ratio is the critical step in
all stoichiometry problems!
A mole ratio converts moles of one
compound in a balanced chemical
equation into moles of another
compound
6
 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 NH3 in
this 1:3:2 ratio of moles
 If
you started with 10 moles of N2 it would
take 30 moles of H2 and would produce 20
moles of NH3
7
For Example
2Mg
+
2 moles
2
:
1 mole
4 moles
3.4 moles
etc
O2
1 mole
1
0.5 mole
2 moles
1.7 moles
etc
→
2MgO
:
2 moles
2
1 mole
4 moles
3.4 moles
etc
8
Let’s Practice
1) How many moles of KClO3 must
decompose to produce 9 moles of oxygen
gas?
2 KClO3
2 moles
2
6 moles
→
2 KCl
:
2 moles
2
+
3 O2
:
3 moles
3
9 moles
9
More practice
2) For the following reaction
2 SO2 + O2 ---> 2 SO3
If you have 4 moles of SO2, how many
moles of O2 would you need?

How many moles of SO3 would you
produce?

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More practice
3) For the following reaction:
2 H2 + O2 ---> 2 H2O
How many moles of H2O are produced
when 5.00 moles of oxygen are used?
If 3.00 moles of H2O are produced, how
many moles of oxygen must be consumed?
How many moles of hydrogen gas must be
used, given the data in problem two?
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More practice
4) For the following reaction:
Fe2O3 + 3 CO ---> 2 Fe + 3 CO2
How
many moles of CO2 are produced if
you start with 10 moles of Fe2O3 ?
How many moles of CO would you want
with 10 moles of Fe2O3 ?
How many moles of Fe would be
produced?
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Let’s make this harder!
5) The reaction below shows the synthesis of
aluminum oxide
3O2 + 4Al → 2Al2O3


If you only had 18 moles of Al, how much
product could you make?
If you wanted to produce 2.4 moles of
product, how many moles of each reactant
would you need?
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Solving #1
Known: 18 mol Al
 Unknown: ? mol Al2O3

3O2 + 4Al → 2Al2O3
18 mol Al x
2 mol Al2O3
4 mol Al
Mole Ratio
= 9.0 mol Al2O3
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Solving #2
Known: 2.4 mol Al2O3
 Unknown: ? mol Al

? Mol O2
3O2 + 4Al → 2Al2O3
2.4 mol Al2O3 x
4 mol Al
2 mol Al2O3
2.4 mol Al2O3 x
3 mol O2
2 mol Al2O3
= 4.8 mol Al
= 3.6 mol O2
15
Work on your own
To solve the problems on the
 Mole to Mole Ratio worksheet
STOICHIOMETRY –
Part II continues
Mass-Mass Relationships
17
Objectives
Take mole-mole stoichiometric
calculations to the next level.
 Interpret balanced chemical equations
in terms of mass.
 Since we don’t have a mole scale, figure
out how much of a product we would
produce from a certain amount of
reactant!

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Recall Our Example
Reaction for the synthesis of aluminum oxide
3O2 + 4Al → 2Al2O3
If you only had 18 moles of Al, how many moles
of product could you make?
18 mol Al x
2 mol Al2O3
4 mol Al
= 9.0 mol Al2O3
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Let’s Take This One Step
Further …
How can would we go about figuring out…
How many grams of Al2O3 would that be?
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How to Relate Grams and Moles?
The molar mass of an element or a
compound is the mass of one mole of
that substance.
 Molar mass is the key to converting
between grams and moles.

Grams
molar mass
Moles
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Molar Mass of Al2O3 =
2(26.98) + 3(16.00) =101.96 g/mol
9 moles
1
x
101.96 grams
1 mole
=
917.64 grams
22
Why Calculate With Mass?



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.
23
General Stoichiometry Steps
1.
Write a balanced equation.
2. Identify your known & unknown.
3. Set up your conversion table.
4. Make sure answer makes sense.
24
Examples
1) Acetylene gas (C2H2) is produced by
adding water to calcium carbide (CaC2)
according to this equation:
CaC2 + H2O →C2H2 + Ca(OH)2
How many grams of C2H2 are produced by
adding water to 5.0 grams of CaC2?
25
Problem #1
grams
5.0
grams
?
grams
_CaC2+ _H2O →_C2H2 + _Ca(OH)2
grams
mole
moles
Examples
2) What mass of BaCl2 is needed to react
completely with 46.8 grams Na3PO4 according
to the equation below?
BaCl2 + Na3PO4  Ba3(PO4)2 + NaCl
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Problem #2
grams
?
46.8
grams
_BaCl2 + _Na3PO4  _Ba3(PO4)2 + _NaCl
grams
mole
moles
Example #3
3) Use the equation below to determine what
mass of FeS must react to form 326 grams of
FeCl2.
FeS + HCl  H2S + FeCl2
30
Problem #3
grams
?
_FeS + _HCl  _H2S
grams
mole
moles
326
grams
+ _FeCl2
Why Is This Important?



We can use this to determine how much of
any reactant we have to add to achieve a
certain amount of a particular product.
We can use this to determine how much of
any product we should get when a certain
amount of a particular reactant is used.
We can use variations of these to
determine % yield or the limiting reagent
in a reaction.
 We
won’t be doing this now – but we will briefly!
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Stop – Work on your own
Work on your own on:
Mass to Mass Stoichiometry problems