Section 2.1 Units and Measurements
• Define SI base units for time, length, mass, and
temperature.
• Explain how adding a prefix changes a unit.
• Compare the derived units for volume and density.
mass: a measurement that reflects the amount of
matter an object contains
Section 2.1 Units and Measurements
base unit
kelvin
second
derived unit
meter
liter
kilogram
density
Chemists use an internationally
recognized system of units to
communicate their findings.
(cont.)
Units
• Système Internationale d'Unités (SI) is an
internationally agreed upon system of
measurements.
• A base unit is a defined unit in a system of
measurement that is based on an object or
event in the physical world, and is
independent of other units. There are seven
base units in SI. All other units are derived
from a combination of these base units.
Units (cont.)
Table on
p. 33 of
your
textbook
Units (cont.)
Table on p. 33
of your
textbook
Prefix
Symbol
Numerical value in base units
Power of 10
equivalent
Giga
G
1,000,000,000
109
Mega
M
1,000,000
106
Kilo
K
1000
103
Hecto
h
100
102
Deka
da
10
101
---
----
1 (base)
100
Deci
d
0.1
10-1
Centi
c
0.01
10-2
Milli
m
0.001
10-3
Micro
μ
0.000001
10-6
Nano
N
0.000000001
10-9
Pico
P
0.000000000001
10-12
Units (cont.)
• The SI base unit of time is the second (s),
based on the frequency of radiation (the
time for a large number of vibrations) given
off by a cesium-133 atom.
• The SI base unit for length is the meter (m),
the distance light travels in a vacuum in
1/299,792,458th of a second.
• The SI base unit of mass is the
kilogram (kg). This weighs
about 2.2 pounds. The kilogram
is an actual physical standard of
platinum and iridium. Scientists
are working to redefine this. Mass
and weight are two different
things.
• Because the kilogram is a large
unit of measure, in the lab we will
usually measure grams.
• How many milligrams are in a
gram?
Mass is a measure of
how much “stuff”
there is….the
amount of matter in
an object. If you go
to the moon, your
weight changes, but
your mass does not.
Units (cont.)
• The SI base unit of temperature is the
kelvin (K).
• Zero kelvin is the point where there is
virtually no particle motion or kinetic
energy, also known as absolute zero.
• You may be more familiar with the
Celsius scale. This scale was set up
based on the boiling and freezing of
water. On this scale, water freezes at
0°C and boils at 100°C. In lab, we will
measure Celsius degrees.
To convert from Celsius to Kelvin, the formula is:
K = °C + 273
Example: Water boils at 100 °C. What temperature is
that on the Kelvin scale?
Example 2: A metal has a temperature of 423 K. What
temperature is that in degrees Celsius?
• Two other temperature scales are Celsius and
Fahrenheit.
• To convert from Celsius to Fahrenheit, the formula is:
°F = 1.8(°C) + 32
Example 3: If the temperature outside is 25 °C, what
temperature is that in Fahrenheit?
Derived Units
• Not all quantities can be measured with SI
base units.
• A unit that is defined by a combination of
base units is called a derived unit.
• For example, the standard unit for speed in
the metric system is m/s. This unit is derived
by dividing the SI unit for length (meter) by
the SI unit for time (second).
Derived Units (cont.)
• Volume is measured in cubic meters (m3), but
this is very large. A more convenient measure
is the liter, or one cubic decimeter (dm3).
Derived Units (cont.)
• Density is a derived unit, g/cm3, the
amount of mass per unit volume.
m
• The density equation is
D=
density = mass/volume.
v
• Example: Find the density of a metal box
that has a volume of 5 cm3 and a mass of
44.6 grams.
• From page 971, what type of box is it?
Section 2.1 Assessment
Which of the following is a derived unit?
A. yard
B. second
C. liter
D
C
A
0%
B
D. kilogram
A. A
B. B
C. C
0%
0%
0%
D. D
Section 2.1 Assessment
What is the relationship between mass
and volume called?
A. density
B. space
D
A
0%
C
D. weight
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. matter
Section 2.2 Scientific Notation and
Dimensional Analysis
• Express numbers in scientific notation.
• Convert between units using dimensional analysis.
quantitative data: numerical information
describing how much, how little, how big, how
tall, how fast, and so on
Section 2.2 Scientific Notation and
Dimensional Analysis (cont.)
scientific notation
dimensional analysis
conversion factor
Scientists often express numbers in
scientific notation and solve problems
using dimensional analysis.
Scientific Notation
• Scientific notation can be used to
express any number as a number between
1 and 10 (the coefficient) multiplied by 10
raised to a power (the exponent).
• Count the number of places the decimal point
must be moved to give a coefficient between
1 and 10.
Scientific Notation (cont.)
• The number of places moved equals the
value of the exponent.
• The exponent is positive when the decimal
moves to the left and negative when the
decimal moves to the right.
800 = 8.0  102
0.0000343 = 3.43  10–5
Example: Express 123 456 in scientific notation.
Scientific Notation (cont.)
• Addition and subtraction
– Exponents must be the same.
– Rewrite values with the same exponent.
– Add or subtract coefficients.
Examples:
Add 1.23 x 103 and 2.46 x 103
Add 2.2 x 102 and 3.33 x 103
(or…use your calculator!)
Scientific Notation (cont.)
• Multiplication and division
– To multiply, multiply the coefficients, then
add the exponents.
– To divide, divide the coefficients, then
subtract the exponent of the divisor from the
exponent of the dividend.
Dimensional Analysis
• Dimensional analysis is a systematic
approach to problem solving that uses
conversion factors to move, or convert,
from one unit to another.
• A conversion factor is a ratio of equivalent
values having different units.
Dimensional Analysis
(cont.)
• Writing conversion factors
– Conversion factors are derived from equality
relationships, such as 1 dozen eggs = 12
eggs.
– Percentages can also be used as
conversion factors. They relate the number
of parts of one component to 100 total parts.
Dimensional Analysis
(cont.)
• Using conversion factors
– A conversion factor must cancel one unit
and introduce a new one.
This is an example that shows how to use
conversion factors in finding how many 8 packs
of soda you would need for a party of 32 people,
assuming each person drinks 2 bottles of soda.
• Examples: Use conversion factors to make the following
conversions:
• 240 s to ms.
• 46 kg to Mg.
Section 2.2 Assessment
What is a systematic approach to problem
solving that converts from one unit to
another?
A. conversion ratio
A
0%
D
D. dimensional analysis
C
C. scientific notation
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. conversion factor
Section 2.2 Assessment
Which of the following expresses
9,640,000 in the correct scientific
notation?
A. 9.64  104
A
0%
D
D. 9.64  610
C
C. 9.64 × 106
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. 9.64  105