1. Chapter 4 Problem Solving in Chemistry
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2. Section 4.1 What Do I Do Now? OBJECTIVES:
List several useful problem-solving skills.

3. Section 4.1 What Do I Do Now? OBJECTIVES:
Describe the three-step problem-solving approach.

4. 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!)

5. 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

6. 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

7. 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

8. Example of Problem Solving
Remember to:
Analyze
Calculate
Evaluate
Sample problem 4-1, page 86
Sample problem 4-2, page 87

9. Section 4.2 Simple Conversion Problems
OBJECTIVES:
Construct conversion factors from equivalent measurements.

10. Section 4.2 Simple Conversion Problems
OBJECTIVES:
Apply the techniques of dimensional analysis to a variety of conversion problems.

11. 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

12. Conversion factors
100 cm
1 m =
100 cm

13. Conversion factors
1 m = 1
100 cm

14. Conversion factors
1 m = 1
100 cm
100 cm =
1 m
1 m

15. Conversion factors
1 m = 1
100 cm 1 =
100 cm
1 m

16. 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

17. Write the possible conversion factors for the following:
Between kilograms and grams
between feet and inches
using qt. = 1.00 L

18. 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 = inches
13 inches x yard = inches

19. 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 = inches
13 inches x yard = 0.36 yards inches

20. Conversion factors
Called conversion factors because they allow us to convert units.
really just multiplying by one, in a creative way.

21. 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!

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

23. Dimensional Analysis
Another measuring system has different units of measure: ft = 1 fathom 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?
Sample 4-5, page 93

24. 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

25. Converting Between Units
Sample 4-6, page 94
Sample 4-7, page 94

26. Section 4.3 More Complex Problems
OBJECTIVES:
Solve problems by breaking the solution into steps.

27. Section 4.3 More Complex Problems
OBJECTIVES:
Convert complex units, using dimensional analysis.

28. 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

29. 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

30. 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