% composition problems

1
The mole road trip itinerary...

• composition problems
• and combustion analysis
• (pp. 94-103)
• Reaction balancing
• (pp. 105-108) cont.
• Reaction stoichiometry predictions, limiting
  yields and yields ( pp. 108-123)

2
How to Reaction balance (pp. 105-108)
Essentially, just try to make elements balance by
trial and error One more trickier one
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More in-class equation balancing practice
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4
__H3PO3 ? ___H3PO4 ___PH3
3
Helpful hint first balance elements that appear
in a single compound on both reactant and product
sides.
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14
12
13
2
____C6H14 ___O2 ? ___CO2 ___H2O
Balance me, please !!! On Board ( a trickier
one)
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Reaction Stoichiometry problems Moles level
3how chemists cook
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Reaction stoichiometry means following..
THE RECIPE
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A simple warm-up with kitchen chemistry
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THE RECIPE
3
2


1
300 g/block
200 g/box
500 g/dozen
1900 g/souffle

• How many boxes of cream to make 4 souffles ?

12
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Cooking and stoichiometry
THE RECIPE
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2


1
300 g/block
200 g/box
500 g/dozen
1900 g/souffle

• 600 grams cheese makes how many souffles ?

2
10
Cooking and stoichiometry
THE RECIPE
3
2


1
300 g/block
200 g/box
500 g/dozen
1900 g/souffle
3000 g

• How many grams of eggs combine with 9 boxes of
  cream ?

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Cooking and stoichiometry
THE RECIPE
3
2


1
300 g/block
200 g/box
500 g/dozen
1900 g/souffle

• 48 eggs combine with how many blocks of cheese?

2
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Simple mol-mol stoichiometry conversions C3H8
5O2--------? 3CO2 4H2O
How many moles of O2 will burn to form 1.2 moles
of CO2 ?
Method 1 factor label way
1) given
2) want
5 mol O2 3 mol CO2
1.2 mol CO2
? mol O2
2
3) Use reaction stoichiometry coefficients in
right ratio to cancel and connectwhich ratio ???
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Mol-mol stoichiometry conversions (continued)
Method 2 mole ratios way
C3H8 5O2--------? 3CO2 4H2O
1.2 mol
?? m
Problem stated visually
How many moles of O2 will burn to form 1.2 moles
of CO2 ?
1) ratio wanted moles in numerator to given in
denominator
5 3
m 1.2
2) Set equal to matching coefficients for
compounds given in reactionwhich ???

1.2 m 1.2 5 1.2 3
2
3)Solve for m
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More complex stoichiometry problem done 2 ways
MW 44 32 44 18 g/mol
C3H8 5O2 ? 3CO2 4H2O
22 grams of C3H8 burned with O2 makes how many
grams of H2O ?
Method 1 factor label
x
22 g C3H8
1 mole C3H8 44 g C3H8
4 mol H2O 1 mole C3H8
18 g H2O 1 mol H2O
x
x
?? g H2O
36
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Method 2 mole ratio way (Board first)
22 grams of C3H8 burned with O2 makes how many
grams of H2O ?
44 32 44 18 g/mol
Given
Problem stated visually
C3H8 5O2 ? 3CO2 4H2O
?? g
22 g
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Method 2 mole ratio way
22 grams of C3H8 burned with O2 makes how many
grams of H2O ?
44 32 44 18 g/mol
Given
Problem stated visually
C3H8 5O2 ? 3CO2 4H2O
?? g
22 g
0) convert all given masses molecule counts to
moles
0.5 mol C3H8
22 g C3H8 1 mol C3H8
44 g C3H8
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Method 2 mole ratio way
22 grams of C3H8 burned with O2 makes how many
grams of H2O ?
44 32 44 18 g/mol
Given
Problem stated visually
C3H8 5O2 ? 3CO2 4H2O
?? g
22 g
0.5 mol C3H8
1) ratio wanted moles m in numerator to given
in denominator
4 1
m mol H2O 0.5 mol C3H8
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Method 2 mole ratio way
22 grams of C3H8 burned with O2 makes how many
grams of H2O ?
44 32 44 18 g/mol
Given
C3H8 5O2 ? 3CO2 4H2O
?? g
22 g
2) Solve for wanted moles
m 0.5
4 1
0.5
0.5
2 mol H2O
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Method 2 mole ratio way
22 grams of C3H8 burned with O2 makes how many
grams of H2O ?
44 32 44 18 g/mol
Given
C3H8 5O2 ? 3CO2 4H2O
?? g
22 g
2 mol H2O
3) Convert wanted moles to wanted final units
(grams)
2 mol H2O 18 g H2O
1 mole H2O
36 g H2O
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Practice, practice, practice.
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Sample Reaction 1 Chemical Stoichiometry Problems
Solved in Detail
Sample reaction 1 C3H8 5O2------ 3CO2 4H2O
44 32 44 18 g/mol

First U try themthen well work through sample
reaction 1 problems a-f the mole ratio way
22
a) mol-mol How many moles of O2 will burn to
form 1.60 moles of H2O?
C3H8 5O2--------? 3CO2 4H2O (BOOM) 44
32 44 18 g/mol

1. 0.8
2. 1.28
3. 2.00
4. 0.20

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C3H8 5O2--------? 3CO2 4H2O (BOOM) 44
32 44
18 g/mol
b) moles to weight How many grams of CO2 are
generated if 0.0757 moles of O2 are burned?
2 g
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c)wt-wt How many grams of O2 are needed to burn
1.1 g C3H8?
C3H8 5O2--------? 3CO2 4H2O (BOOM) 44
32 44 18 g/mol

1. 0.8
2. 4.0
3. 0.25
4. 0.16

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C3H8 5O2--------? 3CO2 4H2O (BOOM) 44
32 44
18 g/mol
d) weight to moles How many moles of H2O form
if 33 g of C3H8 are burned ?
3 mol H2O
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e) How many grams of O2 are needed to form
91022 molecules of H2O ?
C3H8 5O2--------? 3CO2 4H2O (BOOM) 44
32 44 18 g/mol

1. 1.0 g
2. 2.0 g
3. 1.5 g
4. 6.0 g

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C3H8 5O2--------? 3CO2 4H2O (BOOM) 44
32 44 18
g/mol
f) weight to count How many molecules of CO2
form if 1.592 g H2O results ?
41022
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The mole road trip itinerary...

• composition problems and combustion analysis
  (pp. 94-103)
• Reaction balancing (pp. 105-108)
• Reaction stoichiometry predictions, limiting
  yields and yields ( pp. 108-123)

29
Limiting yield and yield calculations are the
last stop on the mole bus trip
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A non-chemical example of a limiting yield
problem

• You have gotten lost on the Ad Dahna desert
  largest desert on the Saudi peninisula. To avoid
  perishing you must reach the nearest oasis which
  is 100 km away. You can walk at maximum 10 km/
  day. You need to consume at least 2 liters of
  water and ½ kg of food per day to walk that
  distance.
• You have in your pack
• 16 liters of water
• 5 kg of food
• Broken cell phone

Whats the maximum distance you can expect to
travel ??
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Food Calculation
5 kg food 10 days ½ kg/day
gt10 days 10 km 100 km ?
day
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Water Calculation
16 liters 8 days 2 liters/day

• 8 days 10 km 80 km ? day

The winner is.always the smaller one it limits.
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3 examples of limiting yield calculationsvia the
cut and try approach (on board)
example 1
5O2 C3H8 ? 3CO2 4H2O
mol 0.33 0.05 ?? mol
Given 0.33 mol O2 and 0.10 mol C3H8, compute the
maximum theoretical yield of CO2 moles
Ans. 0.15 mol CO2 (C3H8 limits)
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The cut try approach to limiting yield
calculations (cont.)
0.18 g H2O (O2 limits)
example 2
5O2 C3H8 ? 3CO2 4H2O
g 4.0 2.2 ? g
MW 32 44 44 18
Given 4.0 grams O2 and 2.2 grams C3H8, compute
the theoretical yield of H2O for the combustion
shown above.
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Cut and try approach example 3

• C12H22O11 12O2
  --------? 12CO2 11H2O
• MW 342 g/mol 32 g/mol 44
  g/mol 18 g/mol
• w(g) 68.40 72.73 ?? g

How many grams of CO2 will be produced if 68.40
grams of sucrose, C12H22O11, is combined
with 72.73 grams of O2 according to the balanced
equation above?
100 g CO2 (O2 limits)
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Guided practice limiting yield problems
2C4H10 13O2--------? 8CO2 10H2O 58
32 44 18 g/mol
Mole-mol-mol a)0.25 moles of C4H10 and 1.4 moles
of O2 are reacted. What is the maximum yield of
CO2 in moles?
0.25 mol C4H10 produces 80.25/2 1 mol CO2
1.4 mol O2 produces 80.25/13 0.15 mol CO2
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Guided practice limiting yield problems (cont.)
2C4H10 13O2--------? 8CO2 10H2O 58
32 44 18 g/mol
Weight-weight-mol b)1 gram of C4H10 and 10 grams
of O2 are reacted. What is the maximum yield in
H2O in moles ?  
1 gram C4H10 1/580.017 mol C4H10
10 gram O2 10/320.312 mol O2
0.017 mol C4H10 produces 100.017/20.085 mol H2O
0.312 mol O2 produces 100.312/13 0.24 mol
H2O
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5.2 Calculate the maximum yield problems (cont.)
2C4H10 13O2--------? 8CO2 10H2O 58
32 44 18 g/mol
Weight-weight-molecules c) 5.8 grams of C4H10 and
160 grams of O2 are reacted. What is the maximum
yield of CO2 in molecule count?
5.8/580.1 mol C4H10
160/325 mol O2
0.1 mol C4H10 produces 80.1/20.4 mol CO2
5 mol O2 produces 85/13 3.07 mol CO2
C4H10 limits
0.461023 molecules CO2 2.41023
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5.2 Calculate the maximum yield problems (cont.)
2C4H10 13O2--------? 8CO2 10H2O 58
32 44 18 g/mol
Weight-molecules-weight d)116 grams of C4H10 and
1.661024 molecules of O2 react. What is the
maximum yield of H2O in grams?
116/582 mol C4H10
1.661024/610231.1 mol O2
2 mol C4H10 produces 102/210 mol H2O
1.1 mol O2 produces 101.1/13 0.85 mol H2O
O2 limits
Multiply down to calculate grams H2O
0.851815.3 g
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U-Try-It on your own Clicker Examples
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2C4H10 13O2--------? 8CO2 10H2O Given
0.5 mol of C4H10 and 6.5 mol of O2, how many
mol of CO2 will form?

1. 8
2. 4
3. 2
4. 1

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2C4H10 13O2--------? 8CO2 10H2O
58 32 44 18 g/mol Given 10 g of C4H10 and
100 g of O2, how many mol of H2O will form?

1. 2.4
2. 0.17
3. 0.86
4. 3.12
5. 0.034

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2C4H10 13O2--------? 8CO2 10H2O
58 32 44 18 g/mol Given 21022 molecules
of C4H10 and 50 g of O2, how many g of CO2 will
form? 1 mol count 61023

1. 5.8 g
2. 0.133 g
3. 4.22 g
4. 1.2 g