Starter
 Convert
3 years to weeks
then to days
then to hours
then to minutes
then to seconds.
2.2 Units of
Measurement
Measurement
 Quantitative
information
 Need a number and a unit (most of time)
 Represents a quantity
 For example: 2 meters



2 is number
Meters is unit
Length is quantity
 Units
compare what is being measured to
a defined measurement standard
SI Measurement
 Le
Systeme International d’Unites : SI
 System of measurement agreed on all
over the world in 1960
 Contains 7 base units
 units are defined in terms of standards of
measurement that are objects or natural
occurrence that are of constant value or
are easily reproducible
 We still use some non-SI units
Important SI Base Units
Quantity
Length
Symbol Unit
l
meter
Abbreviation
m
Mass
m
kilogram
kg
Time
t
second
s
Temperature
T
Kelvin
K
Amount
n
mole
mol
Prefixes

Prefixes are added to the base unit names to
represent quantities smaller or larger
M
mega
106
1,000,000
larger
k
kilo
103
1,000
larger
c
centi
10-2
1/100
smaller
m
milli
10-3
1/1000
smaller
μ
micro
10-6
1/1,000,000
smaller
Mass
 Measure
of the quantity of matter
 SI unit: kg
 use g a lot too
 mass vs. weight



weight is the measure of gravitational pull on
matter
mass does not depend on gravity
on a new planet, mass would be same but
weight could change
Length
 SI
unit: m
 use cm a lot too
 km is used instead of miles for
highway distances and car speeds in
most countries
Derived SI Units
 come
from combining base units
 combine using multiplication or division
Example:
Area: A = length x width
=mxm
= m2
Volume
 amount
of space occupied by object
 SI: m3 = m x m x m
 use cm3 in lab a lot
 non-SI:
1 liter = 1000cm3 = 1000mL
Density

ratio of mass to volume
kg
 SI:
m
3
mass
Density 
volume
 characteristic property of substance (doesn’t
change with amount ) because as volume
increases, mass also increases
 density usually decreases as T increases
exception: ice is less dense than liquid water so
it floats
Example
A sample of aluminum metal has a mass of
8.4 g. The volume is 3.1 cm3. Find the
density.
Known
Unknown
m = 8.4 g
D=?
V = 3.1 cm3
m
8.4 g
g
D 
 2.7 3
3
V 3.1cm
cm
Conversion Factors
 ratio
that comes from a statement of
equality between 2 different units
 every conversion factor is equal to 1
Example:
statement of equality
conversion factor
4quarters  1dollar
1dollar
1
4quarters
Conversion Factors
 can
be multiplied by other numbers
without changing the value of the
number
 since you are just multiplying by 1
4quarters
3dollars 
 12quarters
1dollar
Guidelines for Conversions





always consider what unit you are starting and
ending with
if you aren’t sure what steps to take, write down
all the info you know about the start and end unit
to find a connection
always begin with the number and unit you are
given with a 1 below it
always cancel units as you go
the larger unit in the conversion factor should
usually have a one next to it
Example 1
Convert 5.2 cm to mm

Known: 100 cm = 1 m
1000 mm = 1 m
 Must
use m as an intermediate
1m
1000mm
5.2cm 

 52mm
100cm
1m
Example 2
Convert 0.020 kg to mg

Known: 1 kg = 1000 g
1000 mg = 1 g
 Must
use g as an intermediate
1000 g 1000mg
0.020kg 

 20,000mg
1kg
1g
Example 3
Convert 500,000 μg to kg

Known: 1,000,000 μg = 1 g
1 kg = 1000 g
 Must
use g as an intermediate
1g
1kg
500,000g 

 0.0005kg
1,000,000g 1000 g
Starter 8/12
 Convert
3.76 mm to Mm.
Advanced Conversions
 One
difficult type of conversion deals with
squared or cubed units
 Be sure to square or cube the conversion
factor you are using to cancel all the units
 If you tend to forget to square or cube the
number in the conversion factor, try
rewriting the conversion factor instead of
just using the exponent
Example

Convert:
2000 cm3 to m3
 No intermediate
needed
Known:
100 cm = 1 m
cm3 = cm x cm x cm
m3 = m x m x m
 1m   1m   1m 
3
2000cm  cm  cm 


  0.002m
 100cm   100cm   100cm 
3
OR
 1m 
3
2000cm  

0
.
002
m

 100cm 
3
Advanced Conversions
 Another
difficult type of conversion deals
units that are fractions themselves
 Be sure convert one unit at a time; don’t
try to do both at once
 Work on the unit on top first; then work on
the unit on the bottom
 Setup your work the exact same way
Example

Convert:
350 g/mL to kg/L
 No intermediate
needed
OR
Known:
1000 g = 1 kg
1000 mL = 1 L
350 g
1kg 1000mL
kg


 350
mL 1000 g
1L
L
350 g 1000mL 1kg
kg


 350
mL
1L
1000 g
L
Combination Example

Convert: 7634 mg/m3 to Mg/L
Known:
1000 mg = 1 g
1,000,000 g = 1 Mg
100 cm = 1 m
1 cm3 = 1 mL
1000 mL = 1 L
7634mg
1g
1Mg



3
m
1000mg 1,000,000 g
3
3
1
m
1
cm
1000mL


9 Mg

 7.634 10

 
1L
L
 100cm  1mL