1. 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 in text)
Units are our friends, really!

2. Is a method to convert units.
“Picket Fence”
Is a method to convert units.
Examples include:
Feet Inches
Kilometers  Meters
Kilograms  Grams

3. 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.

4. Principle Behind Dimensional Analysis
You can multiply anything by 1 without changing the physical meaning.

5. 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

6. 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.

7. 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 g
Both cancel out… and become 1.

8. 1 km = 1000 m (A metric equality)
Any equality can be written two ways (conversion factors)
1000 m = 1 km = 1
1 km m
Which one to use?
Depends on which direction you are trying to go.

9. The Rules Write down “given” + unit
Draw a horizontal & vertical line “picket fence”
Write “wanted” / “given” on the fence
Enter proper #s for “wanted units & “given” units
Cancel units & do the math

10. “given” unit = “want” unit For example: Convert 25 km to meters
Use the conversion factor that allows units to cancel.
1000 m or 1 km
1 km m
“given” unit
= “want” unit
picket
fence

11. For example: Convert 25 km to meters (m)
Use the conversion factor that allows units to cancel.
1000 m
1 km
25 km m = m (or 2.5 x 104 m)
Just a little
Scientific
notation
picket
fence
“given” conversion ‘want’
factor

12. Example: Convert 12.5 g to kg
(1)
12.5 g 1 kg
= kg
1000g
(4)
(3)
(2)
(5)
Don’t forget the units

13. See what happens without
UNITS?!?!?

14. Here’s a lady that talks really fast explaining dimensional analysis.
Dimensional Analysis Video Demostration

15. Try: 18.2 km to m

16. Try: How many centimeters are there in 5.54 inches?