Transcript Problem+Solving10

Chapter 3
Problem Solving in Chemistry
3 Methods of Solving Problems
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G M K h da b d c m m m p
Factor Labeling
Formula
Metric Conversions
Move the decimal to the left
G  M  k  h  da  b  d  c  m  m  n  p
Move the decimal to the right
o Every metric unit is different from its neighbor by a factor
of ten from kilo- to milli-, above or below that range, the
units increase or decrease by a factor of 103
o When converting between two units the decimal point is
moved the number of places equal to the distance
between the two unit in the chart above and in the same
direction of movement
Factor Label Method of Conversion
100 cm = 1 m
1 m = 100 cm
100cm
1m
1
1
1m
100cm
Use conversion factors to systematically move from
one unit to the next, cancelling out units on the
diagonal in each step.
Convert
18 m = _______ cm 18m
100 cm
1m
= 1800 cm
Multistep Factor Label Problems
Convert
350 tsp = ______ L
Using the following conversion factors
1 tsp = 5 mL
1 L = 1000 mL
350 tsp
5 mL
1 tsp
1L
1000 mL
= 1.75 L
Multi-step Factor Label Practice
Convert 3 min= ______ms
Use 1 min=60 s and 1000 ms = 1 s
Convert 32oz = _____ g
Use 16 oz=1 lb, 2.2 lb = 1kg, 1kg=1000 g
Multi-dimensional Factor Label Problems
Convert 25 g/mL = ______ kg/dL
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Convert one unit at a time
Recognize that one unit is in the denominator
25 g
1 mL
1 Kg
1000 g
100mL
1 dL
=2.5kg/dL
Multidimensional Factor Label Practice
Convert 85 km/hr = _________m/s
Convert 0.6 L/min = ________ qt/hr
Use 1qt = 1.1L
Factor Label Problems with Squared
or Cubed Units
Convert 25 in2 = ______ cm2
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Recognize that although 1in = 2.54 cm,
1in2 does not equal 2.54 cm2
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Both the unit and its value must be squared
25
in2
2.45 cm
1in
2
= 25
in2
x
6.4516 cm2
1 in2
= 160 cm2
Problem Solving Strategy
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Before you break out your calculator, read the problem all the
way through. Make sure you understand what the question is
asking.
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Write down all of the information you have been given. Keep in
mind, you may be given more facts than you need to use in order
to perform the calculation.
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Write down what it is that you are solving for (desired value).
**Rearrange to create a working equation**
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Write down some useful information like the equation or
equations you will use in order to solve the problem along with
any conversion factors that might be needed
Problem Solving Continued
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Before you plug the numbers into the equations, check the
units required for the equations. You may need to perform
unit conversions before you can apply the equations.
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Once you are certain your units are in agreement, plug the
numbers into the equation and get your answer.
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Round your answer to the correct number of digits and make
sure your answers have units. Use the given values to
determine the number of sig figs in your answer.
***Remember that conversion factors and counting numbers have an
unlimited number of significant figures***
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Ask yourself whether the answer seems reasonable..
Sample Word Problem
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It is known that four hundred pounds of iron metal
occupy a volume of 0.02234 cubic meters.
Calculate the cubic feet of iron in a statue that has
been determined to contain 54.5 kilograms of iron.
Given
400 lbs. Fe/0.02234 m3
54.5 kg Fe
Desired Value
Useful Items
ft3 Fe
D=M/V V=M/D
1kg = 2.2 lbs
1 cubic foot = 2.83 x 104 cm3
1 m3 = 1 x 106 cm3
Sample Word Problem
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The price of gasoline this morning is $3.99/gal. You
are planning a trip out east to visit a friend and
estimate the total distance roundtrip to be 2480 miles.
What will the gasoline cost for the trip? The owner’s
manual stated that our car gets 15 km per liter.
Given
$3.99/gal
2480 mi/round trip
15 km/L
Desired Value
Cost for trip
Useful Items
Millage/”millage rate” = volume
Volume x “cost rate” = total cost
1mi = 1.6 km
1L = 1.1qt
4qt = 1 gal