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Scientific Observation…
Empirical knowledge is gained by conducting experiments
and making observations. There are 2 types of
observations that can be gathered from experiments.
Measurement in Science
Qualitative Observations: Describe the features of an
object or substance using the senses. Ex: Colour, gas
bubbles, odour, precipitate.
Quantitative Observation: requires some sort of measuring
equipment, usually numerical with a unit.
Ex: Temperature (27°C) , Volume (57.4mL).
Qualitative Observations
Measurement
– A Quantitative Observation
• Measured results are required for quantitative observations.
• Various factors will affect your confidence in your measured
results. Such as…
• Give some qualitative
observations for the
picture shown
– Type of measuring equipment used
– Amount to be measured (too large or too small)
– Condition of equipment
All these factors must be controlled in order to increase
confidence and decrease “uncertainty” in your
measurements
The International System of Units
Prefixes in the SI System
The Commonly Used Prefixes in the SI System
Quantity
Name
Symbol
Prefix
Length
Mass
Time
Amount of substance
Thermodynamic temperature
Electric current
Luminous intensity
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 16
meter
kilogram
second
mole
Kelvin
amperes
candela
m
kg
s
mol
K
amps
cd
Symbol
Meaning
Power of 10 for
Scientific Notation
_______________________________________________________________________
1,000,000
106
1,000
103
mega-
M
kilo-
k
deci-
d
0.1
10-1
centi-
c
0.01
10-2
milli-
m
0.001
10-3
micro-
m
0.000001
10-6
nano-
n
0.000000001
10-9
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 118
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Accuracy
vs. Precision
Certainty
of Measurements
Accuracy vs. Precision
• Accuracy refers to the ability of the
measurement to match the “true” value.
How close are you to the real number?
• Precision refers to the ability of a
measurement to be consistently
reproduced
Good accuracy
Good precision
Poor accuracy
Good precision
Random errors:
reduce precision
Example:
• At STP, 5mL of pure water should have a
mass of exactly 5 grams. The following
students weighed a cylinder containing
5mL of pure water three times. Comment
on their accuracy and precision:
Student A
5.0g
5.1g
4.9g
Student B
6.2g
6.1g
6.2g
Student C
6.4g
5.9g
4.2g
Laboratory Equipment
used for approximate measurements
Beaker
Erlenmeyer
Flask
Poor accuracy
Poor precision
Systematic errors:
reduce accuracy
Laboratory Equipment
used for accurate measurements
Burette
Graduated
Cylinder
Volumetric
Flask
Pipette
Estimating the last digit in
measurements:
• The maximum possible accuracty of a
measurement is 1/10 (0.1) times the
smallest division on the measuring
instrument
• Eg. If your ruler’s smallest division is the
tenth’s place, your measurement should
be to the hundredths place
• If your ruler’s divisions are to the one’s,
you estimate to the tenth’s.
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Taking measurements with glassware:
Reading a Meniscus
Measurements are taken to one more
decimal place than the gradations on
the instrument
The graduated cylinder on the
left has scale marks 0.1 mL
apart, so it can be read to the
nearest 0.01 mL.
Reading across the bottom of
the meniscus, a reading of 5.72
mL is reasonable (5.73 mL or
5.71 mL are acceptable, too).
Rules for Rounding
1. If the last digit to be removed is…
a. Less than 5, the preceding digit stays the same.
For example, 1.33 round to 1.3.
b. Equal to or greater than 5, the preceding digit is
increased by 1.
For example, 1.36 rounds to 1.4, and 3.15 rounds to
3.2.
If you have more than one step in a calculation, do not
round until you arrive at the final answer!!!
RULES FOR SIGNIFICANT DIGITS
1. All non-zero digits are significant. (Ex.
367 has 3 sigfigs)
2. All zeros between non-zero digits are
significant. (Ex 307 has 3 sfs)
3. Zeros to the right of the last number
smaller than one are significant. (Ex.
0.300 has 3 sfs)
Significant Digits
• Significant figures are used to show the
accuracy of a measurement.
• All measurements consist of a number of
digits about which we are certain, and a
final digit that has been estimated.
• The measurement must show this
certainty
4. All zeros to the right of the last whole
number are not considered significant
unless measured directly by the
measuring device. (Ex. 6400 km has 2
sfs; 70. g has 2 sfs; 32.00 has 4 sfs)
5. All zeros to the left of a number less than
one, are not significant. (Ex. 0.012 g has
2sfs)
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SIGNIFICANT DIGITS IN CALCULATIONS
6. Exact numbers (numbers derived from
counting) are not considered
measurements. When multiplying or
dividing an uncertain value by an exact
number, the answer has the same place
setting as the measured value. (Ex. 3 x
14.7 mL will be expressed to the tenth)
State the number of significant
digits in the following:
a)
b)
c)
d)
e)
7. When adding or subtracting, the answer is
expressed to the same place setting as the
quantity with the highest place setting, which
means round off your answer to the least
number of decimals in the problem.
8. When multiplying or dividing, the answer
should be rounded off to the same number of
significant digits as the number having the
fewest significant digits.
Metric Conversions
0.00123 g
205 000 g
370.0 g
560. g
1.23x104 g
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