1 Chapter 3 Problem Solving and Conversion Factors

2 Word Problems n The laboratory does not give you numbers already plugged into a formula n You have to decide how to find the answer. n Like word problems in math. n The chemistry book gives you word problems.

3 Problem solving 1 Identify the unknown. Both in words and what units it will be measured in. Both in words and what units it will be measured in. You may need to read the question several times. You may need to read the question several times. 2 Identify what is given Write it down if necessary. Write it down if necessary. Unnecessary information may also be given Unnecessary information may also be given

4 Problem solving 3 Plan a solution Break it down into steps. Break it down into steps. Look up needed information. Look up needed information. Tables Tables Formulas Formulas Constants Constants Equations Equations

5 Problem solving 4 Do the calculations – math 5 Finish by checking Sig Figs Sig Figs Units Units Check your work using an Estimate Check your work using an Estimate Reread the question, did you answer it and is the answer reasonable? Reread the question, did you answer it and is the answer reasonable?

6 Dimensional Analysis n Dimension = unit n Analyze = solve n Using the units to solve the problems. n If the units of your answer are correct, chances are you did the math correctly.

7 Initial and Final Units 1. A person has a height of 2.0 metres. What is that height in centimetres? Initial unit = mFinal unit = _______ Initial unit = mFinal unit = _______ 2) Blood has a density of 0.05 g/mL. If a person lost 30 millilitres of blood at 18°C, how many grams of blood would that be? Initial = millilitresFinal unit = _______

8 Conversion factors n “A ratio of equivalent measurements” n Start with two things that are the same one meter is one hundred centimeters one meter is one hundred centimeters n write it as an equation 1 m = 100 cm 1 m = 100 cm n can set up the ratio two ways = 100 cm = 1 1 m 100 cm1 m

9 Conversion factors n Called conversion factors because they allow us to convert units. n A unique way of writing the number 1, really just multiplying by one n If they are defined quantities, they have unlimited significant figures n big # small unit = small # big unit n 1000 mm = 1 m

10 Write the conversion factors for the following n kilograms to grams n Metres to centimetres n millilitres to litres

11 How many minutes are in 2.5 hours? n Initial unit n 2.5 hr n Conversion Final n factor unit n 2.5 hr x 60 min = 150 min n 1 hr n cancel Answer (2 SF)

12 Learning Check n A rattlesnake is 2.44 m long. How long is the snake in cm? n 1) 2440 cm n 2)244 cm n 3)24.4 cm

13 Solution n A rattlesnake is 2.44 m long. How long is the snake in cm? n 2)244 cm n 2.44 m x 100 cm = 244 cm n 1 m

14 Learning Check n How many seconds are in 1.4 days? n Unit plan: days hr min seconds 2 SF Exact 1.4 day x 24 hr x 60 min x 60 sec 1 day 1 hr 1 min 1 day 1 hr 1 min = 1.2 x 10 5 sec = 1.2 x 10 5 sec

15 Unit Check n What is wrong with the following setup? n 1.4 day x 1 day x 60 min x 60 sec n 24 hr 1 hr 1 min

16 Steps to Problem Solving n Read problem n Identify data n Write down a unit plan from the initial unit to the desired unit n Select conversion factors n Change initial unit to desired unit n Cancel units and check n Do math on calculator n Give an answer using significant figures

17 Learning Check n If the ski pole is 3.0 feet in length, how long is the ski pole in m? 2.54 cm = 1 inch12 inches = 1 foot

18 Solution n 3.0 ft x 12 in x 2.54 cm x 1m = 0.91m n 1 ft 1 in. 100 cm

19 The solutions for some problems contain multi-steps (require more than one calculation to solve). Using Dimensional Analysis can solve this type of problem. Dimensional Analysis 1. Identify the given or known data (information). 2. Identify the unknown. 3. Plan the solution or calculations by either: i.setting up a series of conversion factors OR ii.using a formula. 4. Check your work by canceling out units. 1. Calculate the number of seconds of Chemistry class there is in a week. 1. Calculate the number of seconds of Chemistry class there is in a week. 2. The density of gold is 19.3g. cm 3 cm 3 What is the density of gold expressed inkg? m 3 m 3

20 Practice n If your pace on a treadmill is 65 metres per minute, how many seconds will it take for you to walk a distance of 8450 metres?

21Practice Use conversation factors to solve the following: A pain relief tablet contains 325 mg of ASA. There are 80 tablets in the package of tablets. (a) What is the mass of ASA in grams for each tablet? (b) What is the total mass, in grams, of ASA in the package? (c) A person is permitted to take 1950 mg of ASA per day. How many days will this package last?

22 n T n H n E n N n D