Unit Conversions

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Unit ConversionsUsing 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 doesnt 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

Just a little Scientific notation
picket fence
given conversion want factor
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Example Convert 12.5 g to kg
(1)

• 12.5 g 1 kg
• 0.0125 kg
• 1000g

(3)
(4)
(2)
(5)
Dont forget the units
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See what happens without UNITS?!?!?
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Try 18.2 km to m
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Try How many centimeters are there in 5.54
inches?