1. Chapter 3
Problem Solving

2. 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.

3. Problem solving Identify the unknown.
Both in words and what units it will be measured in.
May need to read the question several times.
Identify what is given
Write it down if necessary.
Unnecessary information may also be given

4. Problem solving Plan a solution The “heart” of problem solving
Break it down into steps.
Look up needed information.
Tables
Formulas
Constants
Equations

5. Problem solving Do the calculations - algebra Finish up Sig Figs Units
Check your work
Reread the question, did you answer it
Is it reasonable?
Estimate

6. Example of Problem Solving
How much heat is needed to raise the temperature of 56.8 g of iron by 65ºC?
Identify the unknown
Heat - calories.
Knowns
Mass, Change in temperature

7. Example of Problem Solving
Plan a solution
Formula Heat = SH x mass x DT
look up SH of Iron = cal/gºC
Do the calculations
heat = cal/gºC x 56.8 g x 65ºC
heat = cal/gºC x g x ºC
heat = 390 cal
Check your work.

8. 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.

9. Conversion factors
100 cm
1 m =
100 cm

10. Conversion factors
1 m = 1
100 cm

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

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

13. 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

14. Write the conversion factors for the following
kilograms to grams
feet to inches
1.096 qt. = 1.00 L

15. 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

16. 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

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

18. Dimensional Analysis Dimension = unit Analyze = solve
Using the units to solve the problems.
If the units of your answer are right, chances are you did the math right.

19. Dimensional Analysis
A ruler is 12.0 inches long. How long is it in cm? ( 1 inch is 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?

20. Multiple units The speed limit is 65 mi/hr. What is this in m/s?
1 mile = 1760 yds
1 meter = yds
65 mi hr
1760 yd
1 mi
1.094 yd
1 m
1 hr
60 min
1 min
60 s

21. Units to a Power 3 How many m3 is 1500 cm3? 1 m 100 cm 1 m 100 cm 1 m

22. 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
Jules Verne wrote a book 20,000 leagues under the sea. How far is this in feet?