Transcript Chapter 2

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One of the key parts of the scientific method
is the ability to make measurements.
If I told you a measurement was 59.7. What
would be your response?
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The metric system is the one used in science.
The units are called SI units-we will see that
not all the units we will use are SI units.
SI base units are listed on p 34.
Quantity
 length
 mass
 time
 current
 temperature
 amt. substance
Unit
meter
kilogram
second
ampere
Kelvin
mole
Symbol
m
kg
s
A
K
mol
3
Name
 mega
 kilo
 deka
 deci
 centi
Symbol
M
k
da
d
c
Multiplier
106
103
10
10-1
10-2
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Name
 milli
 micro
 nano
 pico
 femto
Symbol
m

n
p
f
Multiplier
10-3
10-6
10-9
10-12
10-15
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You must know the following SI prefixes:
Kilo-1000
Deci-0.1
Centi 0.01
Milli 0.001
Others will be provided.
Common Conversion Factors
 Length
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1 m = 39.37 inches
2.54 cm = 1 inch
Volume
◦
◦
1 liter = 1.06 qt
1 qt = 0.946 liter
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The SI base units are used to derive other
units. Some are listed on page 36. One of
the common derived units is volume. The SI
unit for volume is the cubic meter (V=lxwxh)
m3.
This is not a very practical unit to use in the
lab.
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The most commonly
used metric units for
volume are the liter
(L) and the milliliter
(mL).
□ A liter is a cube 1 dm
long on each side.
□ A milliliter is a cube 1
cm long on each side.
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One important physical property of matter is
density .
Density = mass/volume
Every substance has its own unique density.
See p 38 for a list.
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There is some interesting info in the table.
Notice the density of ice: 0.92g/cm3
and for water 0.998g/mL
What do you think this means?
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Since the density formula has 3 variables, 3
types of problems are possible.
D = m/V
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1. given mass and volume-find density
a substance has a mass of 23.2 grams and a
volume of 18.5 cm3. Find its density.
2. given density and volume, find mass (g)
D = m/V so m=D x V
The density of silver is 10.5 g/cm3. Find the
mass of a block of silver with a volume of
40.0cm3.
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3. Given the density and mass, find the
volume of a substance.
D= m/V so V= m/D
Find the volume of a piece of iron that has a
mass of 147grams. ( density of iron = 7.86
g/cm3)
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It is important to be able to convert one unit
to another. We will make use of conversion
factors (also known as unit factors). For
example: how many grams are there in 25
kg?
What you need to know is how many grams
there are in 1 kg. We know (or will know) that
there are 1000g in one Kg (or 1 kg contains
1000 grams.
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25 kg. X 1000 g = 25000 g.
1 kg
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Some for you to try:
a. 1.34 g to kg
b. 15.2 cm to m
c. 2580. mg to kg
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Accuracy refers to the
proximity of a measurement to
the true value of a quantity.
Precision refers to the
proximity of several
measurements to each other.
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If we happen to know the true or accepted
value for a measurement then we can
calculate the per cent error in our
measurement.
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Percent error = (measured value-accepted value) X 100
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accepted value
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Every measurement has some uncertainty
associated with it. See page 46. In every
measurement there is a known or certain
quantity and an estimated quantity.
In every measurement all the numbers are
significant.
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Many measuring instruments allow us to
make an estimate of the last number in a
measurement.
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Piece of Black Paper – with rulers beside the edges
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3
4
5
6
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8
9
10
11
12
13
14
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8
7
6
5
4
3
2
1
1
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Piece of Paper Side B – enlarged
◦ How long is the paper to the best of your ability to measure it?
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12
13
14
23
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6
Piece of Paper Side A – enlarged
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7
◦ How wide is the paper to the best of your ability to
measure it?
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Exact numbers
◦ 1 dozen = 12 things for example
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Accuracy
◦ how closely measured values agree with the
correct value
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Precision
◦ how closely individual measurements agree with
each other
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Significant figures
◦ digits believed to be correct by the person
making the measurement
◦ If you measured it-it’s significant
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Exact numbers have an infinite number of
significant figures
12.000000000000000 = 1 dozen
because it is an exact number
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Significant Figures - Rules
Leading zeroes are never significant
0.000357 has three significant figures
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Trailing zeroes may be significant
must specify significance by how the number is
written
1300 nails - counted or weighed?
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Use scientific notation to remove doubt
2.40 x 103 has ? significant figures
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Try these:
How many significant figures are present in
each of the following measurements
236.5 g
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20.4 cm
_____
970 bricks _____
92.00 kg
____
946025.3709 miles ______
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Multiplication & Division rule
Easier of the two rules
Product has the smallest number of significant
figures of multipliers
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Multiplication & Division rule
Easier of the two rules
Product has the smallest number of significant
figures of multipliers
4.242
x 1.23
5.21766
round off to5.22
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Multiplication & Division rule
Easier of the two rules
Product has the smallest number of significant
figures of multipliers
4.242
2.7832
x 1.23
x 1.4
5.21766
3.89648
round off to5.22
round off to 3.9
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Addition & Subtraction rule
More subtle than the multiplication rule
Answer contains smallest decimal place of the
addends.
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Addition & Subtraction rule
More subtle than the multiplication rule
Answer contains smallest decimal place of the
addends.
3.6923
 1.234
 2.02
6.9463
round off to 6.95
8.7937
 2.123
6.6707
roundoff to 6.671
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