Unformatted text preview: 00 cal in this fashion, this means that we divide by
150,000 cal.
Finally, we note that our final units here are o C1 . This is NOT the unit we
wanted, which was simply o C, so to get our final answer, we take 1. / our answer above,
or 1./6.67x104 o C1 = 1,500 o C. The unit o C1 becomes simply o C on inversion of the
answer, and we get the answer we seek. The very first time I worked out this problem, I
chose mass to start with and my answer was the inverse of that which I sought. Now I
know that I should have started with calories, which would have given the correct units.
However, I chose to start with mass again here to demonstrate that when problems arise,
it is not (typically) the end of reality as we know it.
Practice Problems:
Ready to try a few problems on your own? If so, let's go; following you will find
a series of problems for you to practice on your own. Answers are provided, but YOU
get to get there on your own!
Problem 1: Here's a classic style of problem used to practice FactorLabel. Suppose we
have a farmer who has four cows ready to go to market. He trades the cow for chicken at
the rate of 20 chicken per cow. He trades the chickens for pig at a rate of 15 chicken per
2 pig. On average, each pig weighs 140 lb. The pig he trades for corn, 3 bushels of corn
per pound of pig. The corn is then traded for wheat at 1 bushel of wheat per 2 bushels of
corn. Wheat is premium, so he trades it for rye, 5 bushels of rye per bushel of wheat.
The rye he then gives to a baker, who makes 500 loaves of bread for each bushel. (I'm
making these numbers up, so I have no idea how realistic they are.) The loaves of bread
are given to a pet store who can feed 3 birds per loaf of bread. The birds are used to feed
snakes, and each snake eats an average of 9 birds before being sold for $32.00 each.
How much money did the pet store make from the four cows?'
Answer: Wow, what a horse trader, as it were.
$59,733,333.33. I'm in the wrong business! Dakota State University Turns out the pet store made page 224 of 232 Factor Label Method General Chemistry I and II Lab Manual Problem 2: OK...how about a simpler one, but one that you will need to be able to do
eventually in chemistry? First a onestep calculation: The density of ethyl xanthogenic
ester is 1.085 g/mL. What volume in mL would you need to get 100 g?
Answer: Even if you don't know what ethyl xanthogenic ester is, or even what density is,
you can solve this problem using the factor label method (which turns out to be 92.2 mL).
Problem 3: Another onestep problem: If the molecular weight of the rather long named
compound 5β Pregnan20α  ol3one is 318.50 g/mol, then how many moles
(abbreviated mol) would you have if you had 15.00 g?
Answer: Just what is this stuff? Dunno...don't care, at least not for this problem. Factor
label tells us the answer is 0.04710 mol.
Problem 4: OK, now let's combine the two types of problems, and see how we do. If we
need 0.5000 moles of 3 hexyne, which has a m...
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 Fall '11
 Kumar
 Chemistry, pH, The Crucible, Dakota State University

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