Chapter 3 Problem Solving

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.

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

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

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

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

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.

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.

Conversion factors 100 cm 1 m = 100 cm

Conversion factors 1 m = 1 100 cm

Conversion factors 1 m = 1 100 cm 100 cm = 1 m 1 m

Conversion factors 1 m = 1 100 cm 1 = 100 cm 1 m

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

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

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

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

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

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.

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?

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

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

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?