A way to solve math problems in chemistry Used to convert km to miles, m to km, mol to g, g to mol, etc. To use this we need: 1) desired quantity, 2) given quantity, 3) conversion factors Conversion factors are valid relationships or equities expressed as a fraction E.g. for 1 km=0.6 miles the conversion factor is The factor label method Q. write conversion factors for 1 foot =12 inches Q. what conversion factors can you think of that involve meters?
Conversion factors Conversion factors for 1 ft = 12 in There are almost an infinite number of conversion factors that include meters:
Conversion factors We have looked at conversion factors that are always true. There are conversion factors that are only true for specific questions E.g. A recipe calls for 2 eggs, 1 cup of flour and 0.5 cups of sugar We can use these conversion factors Q - the chemical equation between H 2 and O 2 involves 2 H 2 molecules combining with 1 O 2 molecule to make 2 H 2 O molecules. Write all possible conversion factors
2 molecules H 2 1 molecule O 2 2 molecules H 2 2 molecules H 2 O 2 molecules H 2 1 molecule O 2 2 molecules H 2 O 1 molecule O 2 2 mol H 2 1 mol O 2 2 mol H 2 2 mol H 2 O 2 mol H 2 1 mol O 2 2 mol H 2 O 1 mol O 2 2H 2 + O 2 2H 2 O
The steps to follow Now we are ready to solve problems using the factor label method. The steps involved are: 1.Write down the desired quantity/units 2.Equate the desired quantity to given quantity 3.Determine what conversion factors you can use (both universal and question specific) 4.Multiply given quantity by the appropriate conversion factors to eliminate units you don’t want and leave units you do want 5.Complete the math
Factor label example Q - How many kilometers are in 47 miles? (note: 1 km = miles) First write down the desired quantity # km
Q - How many kilometers are in 47 miles? (note: 1 km = miles) Next, equate desired quantity to the given quantity # km= 47 mi Factor label example
Q - How many kilometers are in 47 miles? (note: 1 km = miles) Now we have to choose a conversion factor # km= 47 mi Factor label example
Q - How many kilometers are in 47 miles? (note: 1 km = miles) What conversion factors are possible? # km= 47 mi 1 km mi 1 km Factor label example
Q - How many kilometers are in 47 miles? (note: 1 km = miles) Pick the one that will allow you to cancel out miles # km= 47 mi 1 km mi 1 km Factor label example
Pick the one that will allow you to cancel out miles Q - How many kilometers are in 47 miles? (note: 1 km = miles) # km= 47 mi 1 km mi 1 km Factor label example
Q - How many kilometers are in 47 miles? (note: 1 km = miles) Multiply given quantity by chosen conversion factor # km= 47 mi 1 km mi 1 km Factor label example
Q - How many kilometers are in 47 miles? (note: 1 km = miles) Multiply given quantity by chosen conversion factor # km= 47 mi x 1 km mi Factor label example
Q - How many kilometers are in 47 miles? (note: 1 km = miles) Cross out common factors # km= 47 mi x 1 km mi Factor label example
Q - How many kilometers are in 47 miles? (note: 1 km = miles) Cross out common factors # km= 47 x 1 km Factor label example
Q - How many kilometers are in 47 miles? (note: 1 km = miles) Are the units now correct? # km= 47 x 1 km Factor label example
Q - How many kilometers are in 47 miles? (note: 1 km = miles) Yes. Both sides have km as units. # km= 47 x 1 km Factor label example
Q - How many kilometers are in 47 miles? (note: 1 km = miles) Yes. Both sides have km as units. # km# km = 47 x 1 km # km Factor label example
Q - How many kilometers are in 47 miles? (note: 1 km = miles) Now finish the math. # km= 47 x 1 km = 75.7 km Factor label example
Q - How many kilometers are in 47 miles? (note: 1 km = miles) The final answer is 75.7 km # km= 47 x 1 km = 75.7 km Factor label example
Summary The previous problem was not that hard In other words, you probably could have done it faster using a different method However, for harder problems the factor label method is easiest
More examples 1.You want to buy 100 U.S. dollars. If the exchange rate is 1 Can$ = 0.65 US$, how much will it cost? # Can$= 100 US$x 1 Can$ 0.65 US$ = Can$
More examples 3.There are 12 inches in a foot, inches in a centimeter, and 3 feet in a yard. How many cm are in one yard? # cm= 1 ydx 3 ft 1 yd = cm x 12 in 1 ft x 1 cm in 4.A chemical reaction requires moles of sodium chloride. How many grams is this? #g NaCl = mol NaClx g NaCl 1 mol NaCl = g NaCl Sodium chloride is NaCl (58.44 g/mol)
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