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Unit Conversions“Using Dimensional Analysis”
We call this the “Picket Fence” method to convert units
(the book calls it the Factor-Label method to convert from one unit to another. (pg. 130-134 in text)
Units are our friends, really!
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“Picket Fence”
Is a method to convert units.
Examples include:
Feet Inches
Kilometers Meters
Kilograms Grams
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Why is Dimensional Analysis Important
• Units are constantly needing to be converted throughout chemistry.
• Stoichiometry (the relationship between quantities of reactants and products in a chemical reaction) problems depend on it.
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Principle Behind Dimensional Analysis
• You can multiply anything by 1 without changing the physical meaning.
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How Do We Multiply By 1
If the same thing is on the top and the bottom of a fraction,
= 1
Then they cancel out… and become 1
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All Equalities = 1
Any time you put two things that are equal to each other
1000g = 1kg
On top and on bottom
1000 g--------- = 1 1 kg
Then they cancel out… and become 1.
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All Equalities = 1
It doesn’t matter which is on top and which is on bottom.
1000 g 1 kg--------- = 1 or --------- = 1 1 kg 1000 g
Both cancel out… and become 1.
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1 km = 1000 m (A metric equality)
Any equality can be written two ways (conversion factors)
1000 m = 1 km = 1 1 km 1000 m
Which one to use? Depends on which direction you are trying to
go.
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The Rules
1. Write down “given” + unit
2. Draw a horizontal & vertical line “picket fence”
3. Write “wanted” / “given” on the fence
4. Enter proper #s for “wanted units & “given” units
5. Cancel units & do the math
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For example: Convert 25 km to meters
Use the conversion factor that allows units to cancel.
1000 m or 1 km
1 km 1000 m
“given” unit= “want” unit
picket fence
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For example: Convert 25 km to meters (m)
Use the conversion factor that allows units to cancel.
1000 m
1 km
25 km 1000 m = 25000 m (or 2.5 x 104 m)
1 km
“given” conversion ‘want’factor
picketfence
Just a littleScientificnotation
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Example: Convert 12.5 g to kg
12.5 g 1 kg= 0.0125
kg1000g
(1)
(2)
(3)(4)
(5)
Don’t forget the units
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See what happens withoutUNITS?!?!?
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• Here’s a lady that talks really fast explaining dimensional analysis.
– Dimensional Analysis Video Demostration
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Try: 18.2 km to m
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Try: How many centimeters are there in 5.54 inches?