Chemistry: SI Units & Chemistry: SI Units & ConversionsConversions

SI Base Units of MeasureSI Base Units of Measurecommonly used in chemistrycommonly used in chemistry

• Time: second (s)• Length,distance: meter (m)• Mass: kilogram (kg)

– In class, we consider the “base unit” to be grams.

• Volume: liter (l)• Temperature: Kelvins (K) note: no degree

symbol– In chemistry lab, we use the Celsius

temperature scale. ( °C)

Derived UnitsDerived Units

• A derived unit is one in which the units are combined mathematically.– Volume: cubic centimeter (cm3)

• (cm x cm x cm)

– Area: square meter (m2)• ( m x m)

– Density: mass volume

3cm

g

Converting Celsius to KelvinConverting Celsius to Kelvin

Kelvins = C + 273 °C = Kelvins - 273

Prefixes for SI UnitsPrefixes for SI Units• Prefixes for units smaller than 1:

– micro () 1 X 10-6 or

– milli (m) 1 X 10-3 or

– centi (c) 1 X 10-2 or

– deci (d) 1 X 10-1 or

• Prefixes for units larger than 1:

– kilo (k) 1 X 103 or 1,000

000,000,11

000,11

1001

101

Conversion Using the Factor-Labeling Conversion Using the Factor-Labeling Method : (Dimensional Analysis)Method : (Dimensional Analysis)

Problem: Convert 32.05 cm to m

Step 1: write down the starting measurement:

number and unit!

cm 32.05

Step 2: Convert from this prefix to the base unit. (This means meters,

liters or grams.)

Xcm 100

m 1

Step 3: Cancel the units that divide each other out.

Step 5 If you have arrived at the target unit, you are done!

Be sure to include the unit and significant figures in

your answer!

= m 0.3205

Step 4: Perform the math… divide by the numbers on the bottom, multiply by the

numbers on the top!

Starting unit

Target unit

Practice!Practice!

Convert 0.00450 km to cm

km 0.00450 Xkm 1

m 1000?

Xm 1cm 100 =

The correct answer is 450 cm, but that’s only

2 sig figs…. ?????

Use scientific notation to correct the sig figs!

4.50 x 102 cm