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!

“Picket Fence”

Is a method to convert units.

Examples include:

Feet Inches

Kilometers Meters

Kilograms Grams

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.

Principle Behind Dimensional Analysis

• You can multiply anything by 1 without changing the physical meaning.

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

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.

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.

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.

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

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

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

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

See what happens withoutUNITS?!?!?

• Here’s a lady that talks really fast explaining dimensional analysis.

– Dimensional Analysis Video Demostration

Try: 18.2 km to m

Try: How many centimeters are there in 5.54 inches?