Chapter 3

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 = 0.106 cal/gºC • Do the calculations heat = 0.106 cal/gºC x 56.8 g x 65ºC heat = 391.352 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. 1 m 100 cm 100 cm 100 cm Conversion factors =

10. Conversion factors 1 m = 1 100 cm

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

12. Conversion factors 1 m = 1 100 cm = 100 cm 1 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 = 1 36 inches • 13 inches x 1 yard = 36 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 = 1 36 inches • 13 inches x 1 yard = 36 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 = 1.094 yds • Pikes peak is 14,110 ft above sea level. What is this in meters?

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

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

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