Ppt 12 Plan
(PS5, 12-18 material)
1. Limiting Reactant Situations
2. % Yield
3.
Molarity (done in lab—Exp. 24; worksheet in next Ppt)
Ppt 12
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Important Realization
(about Stoichiometry Problems thus far)
• In all stoichiometry problems thus far, you
have been TOLD either how much of a
– reactant reacts (g or mol), or
– product forms (g or mol)
A
• You were ASKED:
B
– How much of somebody else was formed or used?
• You then used a mole ratio (from the balanced equation)
to get
─ moles of B that reacted or formed from
moles of A that reacted or formed
2
Example
(from earlier “Quiz” worksheet)
• If 32.6 g of CCl4 reacts according to the
equation below, how many grams of Cl2 will
form?
P4O10 + 5 CCl4  5 CO2 + 4 PCl3 + 4 Cl2
Ppt 12
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How is this problem different?
• If 32.6 g of CCl4 and 11.6 g of P4O10 are
combined, and as much reaction occurs as is
possible according to the equation below, how
many grams of Cl2 will form?
P4O10 + 5 CCl4  5 CO2 + 4 PCl3 + 4 Cl2
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“Limiting Reactant” Situations
• You are given amounts of TWO (or more) reactants
that are present in a container.
• Can all of both reactants react?
– Only if they happen to be present (initially) in the same
ratio in which they react! VERY RARE!
– Usually there will be “leftovers” of one.
– So you cannot assume that what is present all reacts
 You need to figure out who runs out “first” and
who has some leftover!
“limiting reactant”
it limits the amount of reaction
that can occur!
Ppt 12
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How do you figure out which
reactant is limiting?
1) Ratio method. Directly compare mole ratios:
“ratio PRESENT” to “ratio in which they REACT”
– Ratio PRESENT > ratio of REACTION
 “too much of substance in NUMERATOR”
 numerator is in excess; denominator is LR
– Ratio PRESENT < ratio of REACTION
 “too little of substance in NUMERATOR”
 numerator is LR ; denominator is in excess
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How do you figure out which
reactant is limiting?
2) Assumption method. Assume one of the
reactants (A) ALL REACTS. Calculate how
much of the other reactant (B) would be
NEEDED (using mole ratio from equation).
– moles B PRESENT > moles B NEEDED
 “too much of B” (to react with all the A present)
 B is in excess; A is LR
– moles B PRESENT < moles B NEEDED
 “too little of B” (to react with all the A present)
 B is LR ; A is in excess
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**Could go
“grams A” to
“grams of B
needed” also
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How do you figure out which
reactant is limiting?
3) “Which amount of reactants makes the least
amount of product?” method a) Assume one
of the reactants (A) ALL REACTS. Calculate
how much product would be made. b) Do the
same thing assuming all of the other reactant
(B) ALL REACTS.
 Whichever results in fewer products being
made is the L.R.
Ppt 12
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Example
(from "Chemistry, the Molecular Science", Moore, Stanitski, and Jurs (2002, Harcourt) )
CO (g) + 2 H2 (g)

CH3OH (l)
(a) Starting with 12.0 g H2 and 74.5 g CO, which reactant is limiting?
(b) What mass of the excess reactant is left over?
(c) What mass of methanol can be obtained (in theory)?
Ppt 12
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PS Sign-Posting
• The concepts and skills related to
problems 12-17 on PS5 were covered in the
prior section of this PowerPoint. Give those
problems a try now!
Ppt 12
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If masses of
both reactants
are given
(“Limiting
Reactant”
situation)
“Theoretical yield”
(Maximum possible
assuming all of the
LR reacted in the
way you assumed)
Compare the mole ratio
of reactants PRESENT
to the mole ratio of
REACTION (from
equation)
[or use “assumption” or
“which makes the least
amount of products”
method]
Because it all reacted!
Ppt 12
Formed, in theory
OR
moles of other
reactant that
reacted!!
11
Actual vs. Theoretical Yield:
Theory calculates the max, but you rarely get that in
the real world!
• "yield" means "amount of a product“
- it could be in grams or moles
• “theoretical yield" means "maximum amount of product”
- Assumes that as much reaction as possible occurs
according to a given balanced chemical equation).
 no “side” reactions occur
 no losses during isolation (stuck to filter paper, etc.)
- Obtained by doing a stoichiometry calculation using the
balanced equation
• “actual yield" means “amount of product you actually
isolated in the lab after doing a reaction”
Ppt 12
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% Yield:
How well did you do?!
• “% Yield” means “percent of the maximum amount that
you actually got”
partial
% 
x 100
total
partial yield
 % yield 
x 100
total (possible) yield
actual yield
 % yield 
x 100
theoretical yield
• Warning: Read problems carefully! You might not be
asked for % yield!
- If you have any 2 of the 3 “variables” you can calculate the third!
13
Example
(from "Chemistry, the Molecular Science", Moore, Stanitski, and Jurs (2002, Harcourt) )
CO (g) + 2 H2 (g)

CH3OH (l)
What is the percent yield of a reaction in which 5.0 x 103 g of H2
reacts with excess CO to form 3.5 x 103 g CH3OH?
Given: Actual yield (“to form”);
Asked for: % Yield
 Needed: Theoretical yield
Ppt 12
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Stoichiometry Quiz-Revisited
(Chemistry 121 Quiz Used as PRACTICE WORKSHEET on "Early" Stoichiometry)
1. (8 pts) Given the following chemical equation:
P4O10 + 5 CCl4  5 CO2 + 4 PCl3 + 4 Cl2
a. How many moles of CO2 will be formed in theory if 3 moles of P4O10
react?
b.
How many moles of P4O10 would be used up if 1.9 moles of Cl2 were produced?
c. How many moles of PCl3 form in theory if 2.4 moles of CCl4 react?
d. How many moles of CO2 will be formed in theory if 0.247 moles of P4O10
react?
e. How many grams of Cl2 would be formed in theory if 3.2 moles of P4O10
were reacted?
f. If 32.6 g of CCl4 reacts, how many grams of Cl2 form in theory?
***It is implied in all these problems that you are determining a theoretical
yield!***
Ppt 12
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PS Sign-Posting
• The concepts and skills related to
problem 18 on PS5 were covered in the
prior section of this PowerPoint. Give this
problem a try now!
Ppt 12
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