Chapter 3 Problem Solving in Chemistry

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Chapter 3Problem Solving in Chemistry

• G. Holmes Braddock High School
• Mr. Glass

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Section 3.1 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!)

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Section 3.2 Techniques of Problem Solving

• OBJECTIVES
• List five steps used in solving problems.
• Describe the five-step problem-solving approach.

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3.2 Problem solving

• 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

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3.2 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

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Problem solving

• 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

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Example of Problem Solving

• Remember to
• Analyze
• Calculate
• Evaluate
• Example 1, page 62

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Section 3.3Simple Conversion Problems

• OBJECTIVES
• Construct conversion factors from equivalent
  measurements.

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Section 3.4 Dimensional Analysis

• OBJECTIVES
• Apply the techniques of dimensional analysis to a
  variety of conversion problems.

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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

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Conversion factors

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Conversion factors
1
1 m

100 cm
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Conversion factors
1
1 m

100 cm
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Conversion factors
1
1 m

100 cm
100 cm

1
1 m
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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

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Write the possible conversion factors for the
following

• Between kilograms and grams
• between feet and inches
• using 1.096 qt. 1.00 L

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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

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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

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Conversion factors

• Called conversion factors because they allow us
  to convert units.
• really just multiplying by one, in a creative
  way.
• Try Practice Problem 9 on p. 64.

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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

• 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
• Problem Jules Verne wrote a book 20,000 leagues
  under the sea. How far is this in feet?
• Try Practice Problem 10-12 on p. 68

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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

• Do Pr. Problems 13-14 on p. 70.
• Do Pr. Problems 15-17 on p.70.

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Section 3.6More Complex Problems

• OBJECTIVES
• Solve problems by breaking the solution into
  steps.

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Section 3.7More Complex Problems

• OBJECTIVES
• Convert complex units, using dimensional analysis.

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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

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Multistep Problems

• When converting between units, it is often
  necessary to use more than one conversion factor.
• Try Practice Problems 21 22 on p. 74.

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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
• Try Practice Problems 23-25 on p. 76.