Transcript Honors Chapter 3 Measurement

Chemistry
Chapter 3
Scientific Measurement
Qualitative
Measurement
•Gives results in a
descriptive form
•Nonnumeric
Quantitative
Measurement
►Gives
form
results in a definite
►Usually
units
as numbers and
Scientific
Notation
• Shorthand way to
express very large and
very small numbers
Example
4
10
3.6 x
= 3.6 x 10 x 10 x
10 x 10
= 36 000
0.0081 =
-3
8.1 x 10
Direction of decimal
movement
To the left is +
To
the right is -
–Operations with
numbers in
scientific notation
Multiplication
–Multiply the
numbers and then
add the exponents
Division
–Divide the numbers
and subtract the
exponents
Addition and subtraction
Exponent must be
the same to
proceed
Must move the decimal
appropriately and then
adjust the exponent
Then you can solve the
problem
Measured values
only as reliable as
the instrument
used to take the
measurement!
Uncertainty in measurement
Accuracy
Measure
of how
close a measurement
comes to the actual
or true value
Precision
–Measure of how close a
series of measurements
are to one another
Pg. 64 Dartboard example
In class: Pg. 97 #80
Evaluating the accuracy of a
measurement
• Percent error
• Percent error = [error] X 100
accepted value

Error - the
difference between
the accepted value
and the
experimental value
(absolute value)
• Experimental value –
measured in the lab
• Accepted value – correct
value based on reliable
references
• Pg. 65 example
Everyone understand so far?

Good!!!
Significant figures in
measurements (sig figs)
Rules page 66-67
Sample problems
Pg. 68
Sig Figs in Calculations
Rules for rounding Pg. 68
Page 69 Sample
Solving problems with sig figs
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Multiplying and dividing with sig
figs
The answer you get must be
rounded to the same number of sig
figs as the measurement with the
lowest number of sig figs (that you
multiplied or divided)
Example
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Multiply 4.610 feet by 1.7 feet. Express
your answer in correct sig figs
4.610 x 1.7 = 7.837
How do you round it?
4.610 has 4 sig figs
1.7 has 2 sig figs
Round answer to 2 sig figs
Answer = 7.8 square feet
Adding and Subtracting with
sig figs
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When adding or subtracting
measurements, the answer cannot have
more certainly than the least certain
measurement.
Answer must have the same number of
sig figs to the right of the decimal point
as the measurement with the fewest sig
figs to the right of the decimal point
Example

4.271 grams (3 sig figs to the right of
decimal)

2 grams
(0 sig figs to the right of
decimal)
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+ 10.0 grams (1 sig fig to the right of
decimal)
16.271 grams  round 16 grams
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Handout practice – work with a partner!
Grab a calculator
SI System of Units
•Page 73 Units of
measurement
•Table 3.1
• Metric system established
in France in 1790
• SI Adopted by
international agreement in
1960
Prefixes
Page 74
Table 3.2
Length
SI
unit - meter (m)
Pg.
74 Table 3.3
Volume
Space occupied by
any sample of matter
L x W x H
“Derived” unit
Pg. 75 Table 3.4
• Volume of a cube 1m on
each side
• SI unit = m3
• More common to use
Liter (L) = dm3
1 Liter
the volume
occupied by a
cube 10 cm on
each side
10 cm x 10 cm x 10
3
cm = 1000 cm
1000
3
cm
=1L
1 dm = 10 cm
1 L = 1
3
dm
1
mL = 0.001 L
1000 mL = 1 L
1000
3
cm
= 1000 mL = 1 L
•Volumes for
solids, liquids,
gases change
with change in
temperature
Much more dramatic
with gases
Measuring devices
calibrated at 20oC
Room temperature
Mass
• Difference between mass
and weight
• SI unit = Kilogram (kg)
• 1 g = 0.001 kg
• 1000 g = 1 kg
• Pg. 76 Table 3.5
• Will show on board something
special about H2O
Temperature Scales
Celsius
Kelvin
Absolute zero
Kelvin scale explanation
Heat measurement
 calorie
 Joule
 1 cal = 4.184 J
 1J = 0.2390 cal
Unit Conversions
Also called “factor labeling”
How many inches in 2 feet?
How many feet in 36 inches?
You just did a unit
conversion!!!!!!
Look at board
Must use correct
“conversion factor”
• 230 cm = ? m
• Must know that
100 cm =
1m
• Write possible conversion
factors
• 1m
or
100 cm
100 cm
1m
Write the number you are
converting first
Multiply it by the conversion
factor that has the unit you
want your answer to be in
on the TOP
This guarantees that you
will divide or multiply when
you are supposed to.
• 230 cm x
1m
100 cm
=
2.3 m
The top and bottom units
cancel out and the only
unit left is the one you
want you answer to be
in!!!!!
Groups!!
Pg. 84-85 # 32-35
Two step conversions
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4500 cm = ? km
Derived units
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What does “derived” mean?
A derived unit is a measurement
unit created by multiplying or
dividing other units
Miles per hour
words per minute
Area
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Area
Length x width
ft x ft = ft2
ft2 is a derived unit (derived from two
length units)
m x m = m2
m2 is a derived unit (derived from two
length units)
Volume
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Length x width x height
ft x ft x ft = ft3
 m x m x m = m3
 cm x cm x cm = cm3
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Density
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Describes how dense something is
How heavy it is for its size
Density = mass divided by volume
D=M
V
M=DxV
V=M
D
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Since you are dividing two
different measurements, the unit
for density is a DERIVED UNIT.
Derived from a mass measurement
and a volume measurement
g/mL
g/L
Density problem

Calculate the density of a substance with
a mass of 24.3 g and a volume of 32.9 mL.
Use the correct unit and the correct
number of sig figs in your answer.
D=M

V
 D = 24.3 g

32.9 mL
 Ans. = 0.739 g/mL

Problem

What is the volume of an object with a
density of 1.25 g/mL and a mass of 281
g?
V=M

D
 V = 281 g

1.25 g/mL
 g cancels, so units are mL for answer
 V = 225 mL
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Volume of irregularly shaped object
Water displacement
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