6/22/2015
Measurement in Chemistry
Qualitative measurements – Observations that describe
a substance, mixture, reaction, or other process in
WORDS.
Quantitative measurements – Observations that
describe a property with NUMBERS and UNITS.
In 1999, the Mars Climate Orbiter was
lost in the Martian atmosphere because
two separate engineering teams
reported their calculations WITHOUT
UNITS, & failed to communicate with
the units to each other.
The total cost of the mission
was $327.6 MILLION!
UNITS and Quantitative measurements

Numbers often make no sense if we do
not have some sort of reference or
standard to compare them to.

Nearly all numbers MUST be
followed by a unit label.

The unit indicates the standard against which the
number is measured.
UNITS and Quantitative measurements

The metric system is a system of measurement based on
multiples of ten.

In the metric system, a prefix may be added to the base
unit to change the value of the unit by a factor of ten.
The base unit is a reference to the standard.

The English system of measurement is not based on
powers of ten, and is therefore more difficult to use in
calculations.

Scientists almost exclusively work in the metric or SI
system.
1
6/22/2015
Base units: The Système Internationale (SI) base units
are defined from some physically observable and
reproducible quantity. The base units are:
Quantity
Unit
Symbol
Length
meter
Mass
kilogram (gram)
Time
second
S
Temperature
kelvin
K
Amount of a substance
mole
mol
Electric Current
ampere
A
Luminous Intensity
candela
cd
m
kg (g)
Metric Prefixes: The prefixes below MODIFY any of the base or derived
metric units into a power of 10. IN RED: prefixes common to chemistry.
Prefix
Symbol
Multiple
Tera-
T
1012
1,000,000,000,000
Giga-
G
109
1,000,000,000
Mega-
M
106
1,000,000
kilo-
k
103
1,000
hecto-
h
102
100
deka-
dk
101
10
100
1
base unit
Multiple
deci-
d
101
0.1
centi-
c
102
0.01
milli-
m
103
0.001
micro-

106
0.000 001
nano-
n
109
0.000 000 001
pico-
p
1012
0.000 000 000 001
Metric Prefixes: The prefixes below MODIFY any of the base or derived
metric units into a power of 10. IN RED: MEMORIZE THESE PREFIXES.
Prefix
Symbol
Multiple
kilo-
k
103
Multiple
1,000
centi-
c
102
0.01
milli-
m
103
0.001
micro-

106
0.000 001
nano-
n
109
0.000 000 001
2
6/22/2015
Relationships of selected U.S. and Metric Units

In the U.S., many of the everyday measurements we use are
based on the older English system.

We primarily use the metric system for measurements in labs
in the U.S. However it is still often necessary to make some
conversions to the metric system. These conversion factors
will be given if necessary.
Length
1 in = 2.54 cm
Mass
1 lb = 0.4536 kg
Volume
1 qt = 0.9464 L
1 yd = 0.9144 m
1 lb = 16 oz
4 qt = 1 gal
1 mi = 1.609 km
1 oz = 28.35 g
1 mi = 5280 ft
Metric Conversions

First think intuitively about the conversion:
$1 → ? cents
◦ Which unit is larger? Which unit is smaller?
◦ If converting a number from a larger unit to a smaller unit, the
number should become bigger.

Converting from a larger prefix to a smaller one:
◦ Move the decimal to the right:
0.896 m  mm

Converting from a smaller prefix to a larger one:
◦ Move the decimal to the left:
750 mL  cL
DIMENSIONAL ANALYSIS:
A method of solving chemical problems in which the
units are used to set up the solution to a problem.

Dimensional analysis uses a series of ratios (conversion factors) to
convert one unit into another unit.
24 inches
How many feet?
ANSWER
Starting Quantity
24 inches
1 foot
12 inches
= 2 feet
Conversion Factor
3
6/22/2015
Formulating & Making Sense of Conversion Factors

Conversion factors are obtained from a statement of equivalence:
CORRECT
1 mile = 5280 ft
WRONG
5280 miles = 1 ft
1 nm = 10-9 m
10-9 nm = 1 m
Tip: The big number will
always go with the small
unit. The small number
will go with the large
unit.

If converting a number from a larger unit to a smaller unit, the
number should become bigger.
◦ Move the decimal to the left:
14.7 mL  ? L
COMMON MISTAKE
1 mL = 10-3 L
10-3 mL = 1 L
=
=
Steps in dimensional analysis

Identify the conversion to be performed:
GIVEN UNITS  DESIRED UNITS

Setup a Dimensional Analysis table.

Insert conversion factors to ELIMINATE UNWANTED
UNITS and introduce the desired units.

Compute. Be careful to include parentheses where
necessary.
Note that a dimensional analysis table is identical to
multiplying by fractions.
4
6/22/2015
Dimensional Analysis Examples
0.0233 ps
=________ s
9.65 x 108 cg
=________ kg
iClicker Question:
Converting between units of VOLUME
How many cm3 are in 1.5 dm3?
Given: 100 cm = 1 m & 10 dm = 1 m
A. 0.0015 cm3
B. 0.15 cm 3
C. 15 cm 3
D. 150 cm
E. 1500 cm3
Volume Conversion Factors
Common Problem
dm3  cm3
1 dm = 10 cm
(1 dm)3 = (10 cm)3
1 dm3 = 1000 cm3
5
6/22/2015
Compound Units

Volume (space occupied by matter) can be considered a
compound unit. The simplest formula for volume is for the
volume of a box: Volume = (length)∙(width)∙(height)
◦ Consider a box with:
 length = 5.0 cm, width = 3.0 cm, height = 7.0 cm
 Volume = 5.0 cm x 3.0 cm x 7.0 cm = 105 cm3
Just as the numbers are multiplied, so are the units.

Velocity is another common compound unit. It combines
distance and time.
Compound Unit Conversion
Convert: 1.5 in/yr  ? nm/s
Density

Density is a physical property of matter that describes the
relationship between mass and volume of a substance.

Density is an intensive property of matter - A substance will have
a characteristic density that is independent of the amount of the
substance present.

In lay terms, we might say it describes how "heavy" a substance is
(a misuse of the word).
6
6/22/2015
Density from an Atomic Perspective
Density from an Atomic Perspective
5-step Method for Problem-solving
1.
Identify the UNKNOWN in the problem.
2.
Identify the GIVEN quantities and useful information.
3.
Choose the appropriate formulas & conversion
factors.
4.
Plan the solution.
•
•
•
5.
Identify how you will use formulas & conversion factors.
Set up dimensional analysis tables. MAKE SURE UNITS ARE
CONSISTENT.
Isolate unknown variables in formulas.
Substitute the givens (in formulas) and SOLVE.
(Plug & Chug!)
7
6/22/2015
Problem Solving Examples
Pure ethanol has a density of 0.789 g/cm3. What is the
volume of ethanol that must be measured (in fluid ounces)
to equal 14 g?
Given:: 1 fluid ounce ≈ 29.5 mL
Problem Solving Examples
A neutron star has a density of about 100 million tons per
teaspoon (ton/tsp). What volume of a neutron star (in
cm3) would have the same mass as a Boeing 747, which
weighs 358,000 lbs.?
Given: 1 ton = 2000 lbs. & 1 tsp ≈ 4.92 cm3
8