Chapter 2
MEASUREMENTS AND
MOLES
Metric System
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We use the SI (System International)
unit for all scientific work.
A measurement is reported as the
numerical multiple of a standard
unit.
SI Base Unit
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Quantity
Length
Mass
Time
Electric current
Temperature
Chemical amount
Luminous intensity
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Base Unit
Meter (m)
Kilogram (kg)
Second (s)
Ampere (A)
Kelvin (K)
Mole (mol)
Candela (cd)
SI prefixes
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Prefix
G
M
k
d
c
m
µ
n
p
Name
giga
mega
kilo
deci
centi
milli
micro
nano
pico
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Meaning
109
106
103
10-1
10-2
10-3
10-6
10-9
10-12
Extensive and Intensive Properties
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Extensive properties are properties that
depend on the size of the sample example
mass and volume. Doubling the sample
doubles both mass and volume.
Intensive properties are properties that
are independent on the size of the sample
example density ( a ratio of mass and
volume). Another example is temperature.
If we draw 10ml of water from a 1L of
water the temperature remains the same
everytime.
Class Practice
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Find the density of a metal if the water in a
graduated cylinder rises from 50ml to 61.5 ml
when a piece of the metal of mass 35.55 g is
placed in it.
A rock of mass 30.6 g that appears to be beryl is
found in a forest preserve. When it is placed in a
cylinder of water that has been filled to the 60ml
mark, the level of the water rises to 78.4 ml.
A) Find the density of the rock and decide if it
could be beryl. (density of beryl=2.66g/cm3)
B) How many cubic cm would 200 g of rock of
the same substance occupy?
Class Practice On Conversion
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1) 678 ml=
L
2) 46.5 L =
ml
3) 1420 C= K
4) 176µg = kg
5) 16.5 mol=
mmol
6) 56L=
cm3
7) 465mg= µg
8) 650 nanometers=
m.
Significant Figures
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Digits from 1-9 are always significant.
Zeros between two other significant digits
are always significant
One or more additional zeros to the right of
both the decimal place and another
significant digit are significant.
Zeros used solely for spacing the decimal
point (placeholders) are not significant.
Examples Of Significant Figures
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453 kg
All non-zero digits are always significant.
5057 L
Zeros between 2 sig. dig. are significant.
5.00
Additional zeros to the right of decimal
and a significant figures are significant.
0.0071
Placeholders are not significant.
Class Practice
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Report the number of significant
figures in a) 50.00 g
B) 0.00501 m
C) 0.0100 mm
D) 350 ml
Accuracy And Precision
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Precision of a measurement refers to
how close to one another the
repeated measurements are.
Accuracy of a series of
measurements is the closeness of
their average value to the true value.
Errors
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Systematic error are reproducible inaccuracies
that are consistently in the same direction.
Systematic errors are often due to a problem
which persists throughout the entire experiment.
Random errors are statistical fluctuations (in
either direction) in the measured data due to the
precision limitations of the measurement device.
Random errors usually result from the
experimenter's inability to take the same
measurement in exactly the same way to get
exact the same number.
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An accurate measurement is one free
of systematic error.
A precise measurement is one free of
random error.
Mole
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1 mole is the number of atoms in
exactly 12 g of carbon-12. The mass
of a carbon-12 atom is 1.992× 10-23
Number of carbon-12 atoms= 12/ 1.992 ×
10-23 =6.022 × 10-23 (Avogadro’s Constant)
Class Practice
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A sample of vitamin C is known to
contain 2.58 × 10-24 oxygen atoms. How
many moles of O atoms are present in the
sample?
Molar mass
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Mass of the element per mole of its
atom is the molar mass.
The molar mass of a molecular
compound is the mass of the
compound per mole of its molecule.
The molar mass of an ionic
compound is the mass of the
compound per mole of its formula
unit.
Class Practice
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Give the molar masses of the
following compounds:
1. sodium fluoride
2. potassium hydroxide
3. copper (I) chloride
4. manganese (IV) oxide
5. calcium sulfate
6. magnesium phosphate
Average molar mass
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There are two naturally occurring
isotopes of chlorine-35 and chlorine37. The mass of an atom of chlorine35 is 5.807 × 10-23 g and that of an atom
of chlorine-37 is 6.139 × 10-23 g . In a
typical natural sample of chlorine,75.77%
of the sample is chlorine -35 and 24.23%
is chlorine-37.What is the average molar
mass of chlorine?
Empirical formula
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Empirical formula of a compound is a
chemical formula that shows the relative
numbers of atoms of each element, using
the smallest whole numbers of atoms.
Empirical formula for glucose is CH2O tells
us that carbon, hydrogen and oxygen are
present in the ratio of 1:2:1.The molecular
formula for glucose is C6H12O6.
Mass Percent Composition
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To determine the empirical formula
of a compound we start by
measuring the mass of each element
present in the sample. The
composition is usually reported as
mass percentage composition.
Mass%= mass of the element/total
mass ×100%
Class Practice
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The analysis of a sample of
eucalyptal (an ingredient from
eucalyptus tree to cure sore throats)
of total mass 3.16 g gave its
composition as 2.46 g carbon,0.373
g hydrogen, and 0.329 g oxygen.
Determine the mass percentage of
carbon in eucalyptol.
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A compound that helps in the
coagulation of blood has the mass
percentage composition 76.7% C,
7.02% H, and 16.27%N.Determine
the empirical formula of the
compound.