Chapter 4
Problem Solving in Chemistry
Charles Page High School
Dr. Stephen L. Cotton
1
Section 4.1
What Do I Do Now?
 OBJECTIVES:
–List several useful problem-solving
skills.
2
Section 4.1
What Do I Do Now?
 OBJECTIVES:
–Describe the three-step problemsolving approach.
3
Word Problems
 The
laboratory does not give you
numbers already plugged into a
formula
 You have to decide how to get the
answer.
 Like word problems in math.
 The chemistry book gives you word
problems (just like real life!)
4
Problem solving
5
1. ANALYZE
a) Identify the unknown
Both in words and what units it will be
measured in. Write it down!
May need to read the question several
times.
b) Identify what is given (the “known”)
Write it down!
Unnecessary information may also
be given
Problem solving
c) Plan a solution
The “heart” of problem solving
Break it down into steps.
Look up needed information:
Tables
Formulas
Constants, or conversion factors
*Choose an equation
6
Problem solving
2. CALCULATE
doing the arithmetic; use of calculator?
3. EVALUATE
Round off to proper # of sig. figs.
Proper units? Need Scientific Notation?
Check your work!
Reread the question, did you answer it?
Is it reasonable?
Estimate an approximate answer
7
Example of Problem Solving
 Remember
to:
–Analyze
–Calculate
–Evaluate
 Sample
problem 4-1, page 86
 Sample problem 4-2, page 87
8
Section 4.2
Simple Conversion Problems
 OBJECTIVES:
–Construct conversion factors from
equivalent measurements.
9
Section 4.2
Simple Conversion Problems
 OBJECTIVES:
–Apply the techniques of
dimensional analysis to a variety
of conversion problems.
10
Conversion factors
 A “ratio”
of equivalent measurements
 Start with two things that are the same:
one meter is one hundred centimeters
 write it as an equation
1 m = 100 cm
 can divide by each side to come up with
two ways of writing the number 1
11
Conversion factors
1m
100 cm
12
=
100 cm
100 cm
Conversion factors
1m
100 cm
13
=
1
Conversion factors
1m
100 cm
1m
1m
14
=
=
1
100 cm
1m
Conversion factors
1m
100 cm
1
15
=
=
1
100 cm
1m
Conversion factors
 A unique
way of writing the number 1
 In the same system they are defined
quantities so they have unlimited
significant figures
 Equivalence statements always have
this relationship
 big # small unit = small # big unit
 1000 mm = 1 m
16
Write the possible conversion
factors for the following:
 Between
kilograms and grams
 between feet and inches
 using 1.096 qt. = 1.00 L
17
What are they good for?
We can multiply by one creatively to
change the units .
 13 inches is how many yards?
 36 inches = 1 yard.
 1 yard
=1
36 inches
 13 inches x
1 yard
=
36 inches

18
What are they good for?
We can multiply by a conversion factor to
change the units .
 Problem: 13 inches is how many yards?
 Known: 36 inches = 1 yard.
 1 yard
=1
36 inches
 13 inches x
1 yard
=
0.36 yards
36 inches

19
Conversion factors
 Called
conversion factors because
they allow us to convert units.
 really just multiplying by one, in a
creative way.
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Dimensional Analysis
 A way
to analyze and solve problems,
by using units (or dimensions) of the
measurement
 Dimension = unit (such as g, L, mL)
 Analyze = solve
 Using the units to solve the problems.
 If the units of your answer are right,
chances are you did the math right!
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Dimensional Analysis
 A ruler
is 12.0 inches long. How long is it
in cm? ( 1 inch = 2.54 cm)
 in meters?
 A race is 10.0 km long. How far is this in
miles?
– 1 mile = 1760 yds
– 1 meter = 1.094 yds
 Pikes peak is 14,110 ft. above sea level.
What is this in meters?
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Dimensional Analysis

Problem: Jules Verne wrote a book
20,000 leagues under the sea. How
far is this in feet?
 Sample 4-5, page 93

23
Another measuring system has different
units of measure:
6 ft = 1 fathom
100 fathoms = 1 cable length
10 cable lengths = 1 nautical mile
3 nautical miles = 1 league
Converting Between Units
 We
often need to express a
measurement in different units from
the one given or measured.
 Use dimensional analysis!
 Remember to:
–Analyze
–Calculate
–Evaluate
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Converting Between Units
 Sample
4-6, page 94
 Sample 4-7, page 94
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Section 4.3
More Complex Problems
 OBJECTIVES:
–Solve problems by breaking the
solution into steps.
26
Section 4.3
More Complex Problems
 OBJECTIVES:
–Convert complex units, using
dimensional analysis.
27
Multistep Problems
 Many
complex tasks in daily life are
handled by breaking them down
into manageable parts
 Consider cleaning a car:
–vacuum the inside
–wash the exterior
–dry the exterior
–apply a coat of wax
28
Multistep Problems
 When
converting between units, it
is often necessary to use more than
one conversion factor.
 Sample 4-8, page 97
 Sample 4-9, page 98
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Converting Complex Units
 By
complex, we mean units that
may be expressed as a ratio:
–speed is: miles/hour
–gas mileage is: miles/gallon
–density is: g/cm3
 Sample 4-10, page 99
 Sample of ft2 to yd2 and mm3 to cm3
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