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Unit Conversions “Using Dimensional Analysis”

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We call this the “Picket Fence” methodto 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

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.

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

If the same thing is on the top and the bottom of a fraction,

= 1

Then they cancel out… and become 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.

It doesn’t matter which is on top and which is on bottom.

1000 g1 kg

--------- = 1 or --------- = 1

1 kg1000 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.

- 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

For example: Convert 25 km to meters

Use the conversion factor that allows units to cancel.

1000 mor 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

Just a little

Scientific

notation

picket

fence

“given”conversion‘want’

factor

(1)

12.5 g1 kg

= 0.0125kg

1000g

(4)

(3)

(2)

(5)

Don’t forget the units

See what happens without

UNITS?!?!?

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