1 Chapter 3 Problem Solving in Chemistry G. Holmes Braddock High School Mr. Glass

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Chapter 3Chapter 3Problem Solving in ChemistryProblem Solving in Chemistry

G. Holmes Braddock High SchoolG. Holmes Braddock High School

Mr. GlassMr. Glass

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Section 3.1 Word ProblemsSection 3.1 Word Problems The laboratory does not give you The laboratory does not give you

numbers already plugged into a numbers already plugged into a formulaformula

You have to decide how to get the You have to decide how to get the answer.answer.

Like word problems in math.Like word problems in math. The chemistry book gives you word The chemistry book gives you word

problems (just like real life!)problems (just like real life!)

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Section 3.2 Techniques of Section 3.2 Techniques of Problem SolvingProblem Solving

OBJECTIVES:OBJECTIVES:

–List five steps used in solving List five steps used in solving problems.problems.

–Describe the five-step problem-Describe the five-step problem-solving approach.solving approach.

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3.2 Problem solving3.2 Problem solving1. 1. ANALYZEANALYZE

a) Identify the unknowna) Identify the unknown

Both in Both in wordswords and what and what unitsunits it will be it will be measured in. Write it down!measured in. Write it down!

May need to read the question several times.May need to read the question several times.

b) Identify what is given (the “known”)b) Identify what is given (the “known”)

Write it down! Write it down!

Unnecessary information may also Unnecessary information may also be given be given

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3.2 Problem solving 3.2 Problem solving c) Plan a solutionc) Plan a solution The “heart” of problem solvingThe “heart” of problem solving Break it down into steps.Break it down into steps. Look up needed information:Look up needed information: TablesTables FormulasFormulas Constants, or conversion factorsConstants, or conversion factors *Choose an equation*Choose an equation

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Problem solvingProblem solving2.2. CALCULATECALCULATE

doing the arithmetic; use of calculator?doing the arithmetic; use of calculator?3. 3. EVALUATEEVALUATE Round off to proper # of sig. figs.Round off to proper # of sig. figs. Proper Proper unitsunits? Need Scientific Notation?? Need Scientific Notation? Check your work!Check your work! Reread the question, did you answer it?Reread the question, did you answer it? Is it reasonable?Is it reasonable? Estimate an approximate answerEstimate an approximate answer

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Example of Problem SolvingExample of Problem Solving Remember to:Remember to:

–AnalyzeAnalyze–CalculateCalculate–EvaluateEvaluate

Example 1, page 62Example 1, page 62

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Section 3.3Section 3.3Simple Conversion ProblemsSimple Conversion Problems


–Construct conversion factors from Construct conversion factors from equivalent measurements.equivalent measurements.

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Section 3.4 Dimensional Section 3.4 Dimensional AnalysisAnalysis


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

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Conversion factorsConversion factors A “ratio” of equivalent measurementsA “ratio” of equivalent measurements Start with two things that are the same:Start with two things that are the same:

one meter is one hundred centimetersone meter is one hundred centimeters write it as an equationwrite it as an equation

1 m = 100 cm1 m = 100 cm can divide by each side to come up with can divide by each side to come up with

two ways of writing the number 1two ways of writing the number 1

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Conversion factorsConversion factors

100 cm1 m =100 cm 100 cm

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11 m =100 cm

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11 m =100 cm

100 cm=1 m1 m 1 m

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11 m =100 cm

100 cm=1 m

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Conversion factorsConversion factors A unique way of writing the number 1A unique way of writing the number 1 In the same system they are In the same system they are defineddefined

quantities so they have unlimited quantities so they have unlimited significant figuressignificant figures

Equivalence statements always have Equivalence statements always have this relationshipthis relationship

big # big # small unit = small unit = small # small # big unitbig unit 10001000 mm = mm = 11 m m

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Write the possible conversion Write the possible conversion factors for the following:factors for the following:

Between kilograms and gramsBetween kilograms and grams between feet and inchesbetween feet and inches using 1.096 qt. = 1.00 Lusing 1.096 qt. = 1.00 L

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What are they good for?What are they good for? We can multiply by one creatively to We can multiply by one creatively to

change the units .change the units . 13 inches is how many yards?13 inches is how many yards? 36 inches = 1 yard.36 inches = 1 yard. 1 yard = 11 yard = 1 36 36

inchesinches 13 inches x 1 yard 13 inches x 1 yard ==

36 inches36 inches

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What are they good for?What are they good for?

We can multiply by a conversion factor to We can multiply by a conversion factor to change the units .change the units .

Problem: 13 inches is how many yards?Problem: 13 inches is how many yards? Known: 36 inches = 1 yard.Known: 36 inches = 1 yard. 1 yard = 11 yard = 1 36 36

inchesinches 13 inches x 1 yard 13 inches x 1 yard == 0.36 yards0.36 yards

36 inches 36 inches

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Conversion factorsConversion factors Called Called conversion factorsconversion factors because because

they allow us to convert units.they allow us to convert units. really just multiplying by one, in a really just multiplying by one, in a

creative way.creative way.

Try Practice Problem #9 on p. 64.Try Practice Problem #9 on p. 64.

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Dimensional AnalysisDimensional Analysis A way to analyze and solve problems, A way to analyze and solve problems,

by using units (or dimensions) of the by using units (or dimensions) of the measurementmeasurement

Dimension = unit (such as g, L, mL)Dimension = unit (such as g, L, mL) Analyze = solveAnalyze = solve Using the units to solve the problems.Using the units to solve the problems. If the units of your answer are right, If the units of your answer are right,

chances are you did the math right!chances are you did the math right!

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Dimensional AnalysisDimensional Analysis A ruler is 12.0 inches long. How long is it in A ruler is 12.0 inches long. How long is it in

cm? ( 1 inch = 2.54 cm)cm? ( 1 inch = 2.54 cm) in meters?in meters? A race is 10.0 km long. How far is this in A race is 10.0 km long. How far is this in

miles? miles? – 1 mile = 1760 yds1 mile = 1760 yds– 1 meter = 1.094 yds1 meter = 1.094 yds

Pikes peak is 14,110 ft. above sea level. What Pikes peak is 14,110 ft. above sea level. What is this in meters?is this in meters?

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Dimensional AnalysisDimensional Analysis Another measuring system has different Another measuring system has different

units of measure:units of measure: 6 ft = 1 fathom 6 ft = 1 fathom 100 fathoms = 1 cable length100 fathoms = 1 cable length10 cable lengths = 1 nautical mile10 cable lengths = 1 nautical mile 3 nautical miles = 1 league 3 nautical miles = 1 league

Problem: Jules Verne wrote a book Problem: Jules Verne wrote a book 20,000 leagues under the sea. How far is 20,000 leagues under the sea. How far is this in feet?this in feet?

Try Practice Problem #10-12 on p. 68Try Practice Problem #10-12 on p. 68

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Converting Between UnitsConverting Between Units We often need to express a We often need to express a

measurement in different units from measurement in different units from the one given or measured.the one given or measured.

Use dimensional analysis!Use dimensional analysis! Remember to:Remember to:

–AnalyzeAnalyze–CalculateCalculate–EvaluateEvaluate

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Converting Between UnitsConverting Between Units

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

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Section 3.6Section 3.6More Complex ProblemsMore Complex Problems


–Solve problems by breaking the Solve problems by breaking the solution into steps.solution into steps.

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Section 3.7Section 3.7More Complex ProblemsMore Complex Problems


–Convert complex units, using Convert complex units, using dimensional analysis.dimensional analysis.

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Multistep ProblemsMultistep Problems Many complex tasks in daily life are Many complex tasks in daily life are

handled by breaking them down into handled by breaking them down into manageable partsmanageable parts

Consider cleaning a car:Consider cleaning a car:–vacuum the insidevacuum the inside–wash the exteriorwash the exterior–dry the exteriordry the exterior–apply a coat of waxapply a coat of wax

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Multistep ProblemsMultistep Problems

When converting between units, it When converting between units, it is often necessary to use more than is often necessary to use more than one conversion factor.one conversion factor.

Try Practice Problems # 21 & 22 on Try Practice Problems # 21 & 22 on p. 74.p. 74.

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Converting Complex UnitsConverting Complex Units

By complex, we mean units that By complex, we mean units that may be expressed as a ratio:may be expressed as a ratio:

–speed is: miles/hourspeed is: miles/hour

–gas mileage is: miles/gallongas mileage is: miles/gallon

–density is: g/cmdensity is: g/cm33

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

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1 Chapter 3 Problem Solving in Chemistry G. Holmes Braddock High School Mr. Glass