Keys to the Study of Chemistry

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Keys to the Study of Chemistry

• Chapter 1

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Chapter 1 Keys to the Study of Chemistry
1.1 Some Fundamental Definitions 1.2 Chemical
Arts and the Origins of Modern Chemistry 1.3 The
Scientific Approach Developing a Model 1.4
Chemical Problem Solving 1.5 Measurement in
Scientific Study 1.6 Uncertainty in Measurement
Significant Figures
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CHEMISTRY
is the study of matter, its properties, the
changes that matter undergoes, and the
energy associated with these changes.
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Definitions
Matter
anything that has mass and volume -the stuff of
the universe books, planets, trees, professors,
students
Composition
the types and amounts of simpler substances that
make up a sample of matter
Properties
the characteristics that give each substance a
unique identity
Chemical Properties Those which the substance
shows as it interacts with, or transforms into,
other substances (such as flammability,
corrosiveness)
Physical Properties Those which the substance
shows by itself without interacting with another
substance (such as color, melting point, boiling
point, density)
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Sample Problem 1.1
Distinguishing Between Physical and Chemical
Change
(a) Frost forms as the temperature drops on a
humid winter night.
(b) A cornstalk grows from a seed that is watered
and fertilized.
(c) Dynamite explodes to form a mixture of gases.
(d) Perspiration evaporates when you relax after
jogging.
(e) A silver fork tarnishes in air.
PLAN
Does the substance change composition or just
change form?
SOLUTION
(a) physical change
(b) chemical change
(c) chemical change
(d) physical change
(e) chemical change
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Figure 1.2
The Physical States of Matter
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Energy is the capacity to do work.
energy due to the position of the object or
energy from a chemical reaction
energy due to the motion of the object
Figure 1.3
Potential and kinetic energy can be
interconverted.
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Energy is the capacity to do work.
Figure 1.3(continued)
Potential Energy
energy due to the position of the object or
energy from a chemical reaction
Kinetic Energy
energy due to the motion of the object
Potential and kinetic energy can be
interconverted.
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Scientific Approach Developing a Model
Natural phenomena and measured events
universally consistent ones can be stated as a
natural law
Observations
Hypothesis
Tentative proposal that explains observations
Procedure to test hypothesis measures one
variable at a time
Experiment
Set of conceptual assumptions that explains data
from accumulated experiments predicts related
phenomena
Model (Theory)
Further Experiment
Tests predictions based on model
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Sample Problem 1.2
Converting Units of Length
PLAN
Know length (in cm) of wire and cost per length
(in ft) Need to convert cm to inches and inches
to ft followed by finding the cost for the length
in ft.
SOLUTION
length (cm) of wire
Length (in) length (cm) x conversion factor
2.54 cm 1 in
325 cm x
128 in
length (in) of wire
12 in 1 ft
Length (ft) length (in) x conversion factor
length (ft) of wire
128 in x
10.7 ft
1 ft 0.15
Price () length (ft) x conversion factor
Price () of wire
10.7 ft x
1.60
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Table 1. 2
SI - Base Units
Physical Quantity
Unit Name
Abbreviation
mass
kg
kilogram
meter
length
m
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Common Decimal Prefixes Used with SI Units
Table 1.3
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Common SI-English Equivalent Quantities
Table 1.4
English to SI Equivalent
English Equivalent
Quantity
SI Unit
SI Equivalent
Length
1 kilometer (km)
1000 (103) m
0.6214 mi
1 mi 1.609 km
1 meter (m)
100 (102) cm
1.094 yd
1 yd 0.9144 m
1000 (103) mm
39.37 in
1 ft 0.3048 m
1 centimeter (cm)
0.01 (10-2 ) m
0.3937 in
1 in 2.54 cm (exactly)
Volume
1,000,000 (106) cm3
35.31 ft3
1 cubic meter (m3)
1 ft3 0.02832 m3
1 cubic decimeter (dm3)
1000 cm3
0.2642 gal 1.057 qt
1 gal 3.785 dm3
1 qt 0.9464 dm3
1 cubic centimeter (cm3)
0.001 dm3
0.03381 fluid ounce
1 qt 946.4 cm3
1 fluid ounce 29.57 cm3
Mass
1 kilogram (kg)
1000 g
2,205 lb
1 lb 0.4536 kg
1 gram (g)
1000 mg
0.03527 oz
1 lb 453.6 g
1 oz 28.35 g
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Sample Problem 1.3
Determining the Volume of a Solid by Displacement
of Water
PLAN
The volume of galena is equal to the change in
the water volume before and after submerging the
solid.
volume (mL) before and after addition
SOLUTION
(24.5 - 19.9)mL volume of galena
volume (mL) of galena
4.6 mL x
4.6 cm3
volume (cm3) of galena
volume (L) of galena
4.6 mL x
4.6x10-3 L
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Sample Problem 1.4
Converting Units of Mass
PLAN
The sequence of steps may vary but essentially
you have to find the length of the entire cable
and convert it to mass.
SOLUTION
length (km) of fiber
8.84 x 106 m
length (m) of fiber
1.05 x 104 lb
mass (lb) of fiber
mass (kg) of cable
mass (lb) of cable
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Some Interesting Quantities
Figure 1. 10
Length Volume Mass
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Densities of Some Common Substances
Table 1.5
Substance Physical State
Density (g/cm3)
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Sample Problem 1.5
Calculating Density from Mass and Length
PLAN
Density is expressed in g/cm3 so we need the mass
in grams and the volume in cm3.
SOLUTION
lengths (mm) of sides
1.49 g
1.49x103 mg x
mass (mg) of Li
lengths (cm) of sides
20.9 mm x
2.09 cm
Similarly the other sides will be 1.11 cm and
1.20 cm, respectively.
mass (g) of Li
volume (cm3)
2.09 x 1.11 x 1.20 2.76 cm3
density (g/cm3) of Li
density of Li
0.540 g/cm3
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Some Interesting Temperatures
Figure 1.11
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The Freezing and Boiling Points of Water
Figure 1.12
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Temperature Scales and Interconversions
Kelvin ( K ) - The absolute temperature scale
begins at absolute zero and only has positive
values.
Celsius ( oC ) - The temperature scale used by
science, formally called centigrade and most
commonly used scale around the world. Water
freezes at 0oC and boils at 100oC.
Fahrenheit ( oF ) - Commonly used scale in the US
for weather reports. Water freezes at 32oF and
boils at 212oF.
T (in K) T (in oC) 273.15 T (in oC) T (in
K) - 273.15
T (in oF) 9/5 T (in oC) 32 T (in oC) T
(in oF) - 32 5/9
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Sample Problem 1.6
Converting Units of Temperature
(a) If normal body temperature is 98.60F, does
the child have a fever?
(b) What is the childs temperature in kelvins?
PLAN
We have to convert 0C to 0F to find out if the
child has a fever and we use the 0C to kelvin
relationship to find the temperature in kelvins.
SOLUTION
(a) Converting from 0C to 0F
(b) Converting from 0C to K
38.70C 273.15 311.8K
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The Number of Significant Figures in a
Measurement Depends Upon the Measuring Device
Figure 1.14
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Rules for Determining Which Digits are
Significant
except zeros that are used only to position the
decimal point.
All digits are significant

• Make sure that the measured quantity has a
  decimal point.
• Start at the left of the number and move right
  until you reach the first nonzero digit.
• Count that digit and every digit to its right as
  significant.

Zeros that end a number and lie either after or
before the decimal point are significant thus
1.030 mL has four significant figures, and 5300.
L has four significant figures also. Numbers
such as 5300 L are assumed to only have 2
significant figures. A terminal decimal point is
often used to clarify the situation, but
scientific notation is the best!
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Sample Problem 1.7
Determining the Number of Significant Figures
(b) 0.1044 g
(a) 0.0030 L
(c) 53.069 mL
(e) 57,600. s
(d) 0.00004715 m
(f) 0.0000007160 cm3
PLAN
Determine the number of sf by counting digits and
paying attention to the placement of zeros.
SOLUTION
2sf
4sf
5sf
(f) 7.160x10-7 cm3
(d) 4.715x10-5 m
4sf
(e) 5.7600x104 s
5sf
4sf
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Rules for Significant Figures in Answers
1. For addition and subtraction. The answer has
the same number of decimal places as there are
in the measurement with the fewest decimal
places.
Example adding two volumes
106.78 mL 106.8 mL
Example subtracting two volumes
863.0879 mL 863.1 mL
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Rules for Significant Figures in Answers
2. For multiplication and division. The number
with the least certainty limits the certainty of
the result. Therefore, the answer contains the
same number of significant figures as there are
in the measurement with the fewest significant
figures.
Multiply the following numbers
9.2 cm x 6.8 cm x 0.3744 cm
23.4225 cm3 23 cm3
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Issues Concerning Significant Figures
be sure to correlate with the problem
FIX function on some calculators
graduated cylinder lt buret pipet
60 min 1 hr
numbers with no uncertainty
1000 mg 1 g
These have as many significant digits as the
calculation requires.
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Rules for Rounding Off Numbers
1. If the digit removed is more than 5, the
preceding number increases by 1. 5.379 rounds to
5.38 if three significant figures are retained
and to 5.4 if two significant figures are
retained.
2. If the digit removed is less than 5, the
preceding number is unchanged. 0.2413 rounds to
0.241 if three significant figures are retained
and to 0.24 if two significant figures are
retained.
3.If the digit removed is 5, the preceding number
increases by 1 if it is odd and remains unchanged
if it is even. 17.75 rounds to 17.8, but 17.65
rounds to 17.6. If the 5 is followed only by
zeros, rule 3 is followed if the 5 is followed
by nonzeros, rule 1 is followed 17.6500 rounds
to 17.6, but 17.6513 rounds to 17.7
4. Be sure to carry two or more additional
significant figures through a multistep
calculation and round off only the final answer.
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Sample Problem 1.8
Significant Figures and Rounding
PLAN
In (a) we subtract before we divide for (b) we
are using an exact number.
SOLUTION
2.104 cm2
4.16 g/ cm3
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Precision and Accuracy Errors in Scientific
Measurements
Precision Refers to reproducibility or how
close the measurements are to each other
Accuracy Refers to how close a measurement is to
the actual value
Systematic error Values that are either all
higher or all lower than the actual value
Random Error In the absence of systematic error,
some values that are higher and some that are
lower than the actual value
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Figure 1.16
Precision and Accuracy in the Laboratory
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Precision and Accuracy in the Laboratory
Figure 1.16 continued