Problem Solving in Chemistry

1. Problem Solving in Chemistry • Dimensional Analysis • Used in _______________ problems. • *Example: How many seconds are there in 3 weeks? • A method of keeping track of the_____________. • Conversion Factor • A ________ of units that are _________________ to one another. • *Examples: 1 min/ ___ sec (or ___ sec/ 1 min) • ___ days/ 1 week (or 1 week/ ___ days) • 1000 m/ ___ km (or ___ km/ 1000 m) • Conversion factors need to be set up so that when multiplied, the unit of the “Given” cancel out and you are left with the “Unknown” unit. • In other words, the “Unknown” unit will go on _____ and the “Given” unit will go on the ___________ of the ratio. conversion units ratio equivalent 60 60 7 7 1 1 top bottom

2. How to Use Dimensional Analysis to Solve Conversion Problems • Step 1: Identify the “________”. This is typically the only number given in the problem. This is your starting point. Write it down! Then write “x _________”. This will be the first conversion factor ratio. • Step 2: Identify the “____________”. This is what are you trying to figure out. • Step 3: Identify the ____________ _________. Sometimes you will simply be given them in the problem ahead of time. • Step 4: By using these conversion factors, begin planning a solution to convert from the given to the unknown. • Step 5: When your conversion factors are set up, __________ all the numbers on top of your ratios, and ____________ by all the numbers on bottom. Given Unknown conversion factors multiply divide If your units did not ________ ______ correctly, you’ve messed up! cancel out

3. Practice Problems: • How many hours are there in 3.25 days? • (2) How many yards are there in 504 inches? • (3) How many days are there in 26,748 seconds? 24 hrs 3.25 days 78 hrs = x 1 day 1 ft 1 yard 504 in. 14 yards x = x 12 in. 3 ft 1 min 1 hr 1 day 26,748 sec 0.30958 days x x x = 60 sec 60 min 24 hrs

4. Converting Complex Units • A complex unit is a measurement with a unit in the _____________ and ______________. • *Example: 55 miles/hour 17 meters/sec 18 g/mL • To convert complex units, simply follow the same procedure as before by converting the units on ______ first. Then convert the units on __________ next. • Practice Problems: (1) The speed of sound is about 330 meters/sec. What is the speed of sound in units of miles/hour? (1609 m = 1 mile) • (2) The density of water is 1.0 g/mL. What is the density of water in units of lbs/gallon? (2.2 lbs = 1 kg) (3.78 L = 1 gal) • (3) Convert 33,500 in2 to m2 (5280 ft = 1609 m) (12 inches = 1 foot) numerator denominator top bottom 330m 1 mile 3600 sec 738 miles/hr x = x sec 1609 m 1 hr 1.0 g 1 kg 2.2 lbs 1000 mL 3.78 L 8.3 lbs/gal x x x x = mL 1000 g 1 kg 1 L 1 gal 1 ft 2 1609 m 2 33,500 in2 21.6 m2 x x = 12 in. 5280 ft