Unit 2.3 Measurements SI Units
Teacher: Dr. Van Der Sluys
Objectives
Use the SI system.
*
Know the SI base units.
*
State rough equivalents for the SI base units in the English system.
*
Read and write the symbols for SI units.
*
Recognize unit prefixes and their abbreviations.
*
Build derived units from the basic units for mass, length, temperature, and time.
*
Convert measurements from SI units to English, and from one prefixed unit to
another.
*
Use derived units like density and speed as conversion factors.
*
Use percentages, parts per thousand, and parts per million as conversion factors.
Use and report measurements carefully.
*
Consider the reliability of a measurement in decisions based on measurements.
*
Clearly distinguish between precision and accuracy
*
exact numbers and measurements
*
systematic error and random error
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Non-SI units Encountered in Chemistry
Non SI unit
Unit type
SI conversion
Notes
liter (L)
Volume
1 L = 1000 cm3
1 quart = 0.946 L
Angstrom (Å)
Length
1 Å = 10-10 m
typical radius of an
atom
atomic mass unit
(amu)
Mass of atoms
1 amu =
1.66054Å~10- 27 kg
about the mass of a
proton or neutron
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Scientific (Exponential)
Notation
•
•
•
•
•
Any number can be express as the product of two other
numbers. It is useful if one of these numbers is a factor of 10.
Example 24 = 2.4 x 10
An exponent is an integer that tells you how many time the
base number is multiplied by itself. Example 100 = 10 x 10 =
102
These two concepts can be used together and forms the basis
for scientific notation, which is used to express very large or
small numbers. Example 240 = 2.40 x 100 = 2.40 x 102
When using scientific notation always place the decimal point
after the first non-zero digit.
Numbers that are larger than 10 with have positive exponents
and numbers that are smaller than 1 will have negative
exponents.
Scientific Notation Examples
• Convert the following numbers to
scientific notation.
50000000 =
0.00087 =
0.967 =
432 =
Commonly used SI prefixes
Prefix
Meaning
Abbreviation
Exponential
Notation
Giga-
billion
G
109
Mega-
million
M
106
kilo-
thousand
k
103
centi-
hundredths of
c
10- 2
milli-
thousandths of
m
10- 3
micro-
millionths of
µ
10- 6
nano-
billionths of
n
10- 9
pico-
trillionths of
p
10=12
3
1 quart = 0.946 L
Using Prefixes to Report
Measured Values
• If a measured number is placed in scientific
notation and the factor of 10n corresponds to
the exponetial notation of one of the prefixes
it is sometimes useful to replace the
exponential notation with the prefix.
• How many kilometers (km) are in 5000
meters (m)?
5000 m = 5 x 103 m
Since 103 = kilo
5000 m = 5 km
Use of Prefixes Examples
• Convert the following units
0.034 m to cm
4000 g to kg
1.5 ms to s
2.5 µg to g
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The Dimensional Analysis Approach
• Use conversion factors like ratios
1 gallon = 4 quarts and 1 quart = 0.946 L
15 gal x 4 qt x 0.946 L
1 gal
1 quart
=
56.8 L
1 m = 39.36 in
1 mile = 5 280 feet
1 quart = 0.946 L
Prefixes and Dimensional
Analysis
• Convert the following using
dimensional analysis
25 g to kg (1 kg = 103 g)
50 pm to cm (1 pm = 10-12m; 1 cm = 10-2 m)
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