Chemical Quantities
10.1
Key Question
How can you convert among the count, mass,
and volume of something?
Knowing how the count, mass, and volume of an
item relate to a common unit allows you to
convert among these units.
ROCK ME AVOGADRO!
https://www.youtube.com/watch?v=13WUqWd_Yk8
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A mole (mol) of a substance is 6.02 x 1023
representative particles (atom, molecule, ion, or
formula unit) of that substance and is the SI unit
for measuring the amount of a substance
The number of representative particles in a mole,
6.02 x 1023, is called Avogadro’s number (NA)
It was named in honor of the Italian scientist
Amedeo Avogadro, who helped clarify the
difference between atoms and molecules
Representative particles refer to the species present
in a substance, usually atoms, molecules, or
formula units
Key Question
How do chemists count the number of atoms,
molecules, or formula units in a substance?
The mole allows chemists to count the number of
representative particles in a substance.
Table 10.1 summarizes the relationship
between representative particles and moles of
substances.
1 mol = 6.02 x 1023 representative particles
Representative particles being an atom, a molecule, an ion,
or a formula unit!
A mole is always the same number, but 1-mole samples of
different substances will have different masses
Example: 1 mol of C and 1 mol of Mg
C has a mass of ~12 amu
Mg has a mass of ~24 amu
The mass of a single atom of an element is
numerically equal to the mass of 1 mol of that
element.
Therefore, the molar mass (g/mol) of any
substance is always equal to its formula or
molecular weight.
PROBLEM SOLVING
Step 1: Analyze the problem
Step 2: Develop a plan for solving the problem
Step 3: Solve the problem
Step 4: Check the solution
Converting between Number of Particles and Moles
IN CLASS ONLY!
Converting between Number of Particles and Moles
*To find the number of atoms in a given number of
moles of a compound, you must first determine
the number of representative particles!
IN CLASS ONLY!
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Key Question
How do you determine the molar mass of an
element and of a compound?
Of an element-The atomic mass of an element
expressed in grams is the mass of a mole of the
element.
The mass of a mole of an element is its molar mass.
Of a compound-To calculate the molar mass of a
compound, find the number of grams of each
element in one mole of the compound. Then add
the masses of the elements in the compound.
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C
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H
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12.011 g/mol
1.0079 g/mol
S
Al
He
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H2O
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CO

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76.13 g/mol
CCl4

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28.010 g/mol
CS2

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18.015 g/mol
153.823 g/mol
S2Cl2

135.026 g/mol
10.2
Key Question
How do you convert the mass of a substance to the
number of moles of the substance?
Use the molar mass of an element or compound to convert
between the mass of a substance and the moles of the
substance
Conversion Factor
Molar mass = 1 mol
;therefore,
mass
1 mol
and
1 mol
mass
IN CLASS ONLY!
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Avogadro’s hypothesis states that equal volumes of
gases at the same temperature and pressure contain
equal numbers of particles
The particles that make up different gases are not the
same size
The volume of a gas varies with a change in
temperature or a change in pressure; therefore, the
volume of a gas is usually measured at a standard
temperature and pressure
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Standard Temperature and Pressure (STP) means a
temperature of 273 K (0 oC), 1.0 atm (101.3 kPa, and 22.4 L
At STP 1 mol of any gas occupies a volume of 22.4 L
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22.4 L is called the molar volume of a gas
How do you convert the volume of a gas at STP to the
number of moles of the gas?
The molar volume is used to convert between the number
of moles of gas and the volume of the gas at STP.
Conversion Factor
22.4 L = 1 mol at STP
;therefore,
22.4 L
1 mol
and
1 mol
22.4 L
IN CLASS ONLY!
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Density is the ratio of the mass of an object to its volume
Key Question
What determines the density of a substance?
Density is an intensive property that depends only on the
composition of a substance
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The SI units of density is g/cm3 or g/L
The SI unit of mass is g
The SI unit of volume is cm3 or L
The density of a gas is measured in g/L
The density of a gas at STP and the molar volume at STP can
be used to calculate the molar mass of the gas, and the molar
mass of a gas and the molar volume at STP can be used to
calculate the density of a gas at STP
IN CLASS ONLY!
g

m

rp
(grams, moles, repres. par.)
and
Vice Versa
10.3
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Percent composition is the percent by mass of
each element in the compound
How do you calculate the percent composition
of a compound?
The percent by mass of an element in a compound
is the number of grams of the element divided
by the mass in grams of the compound,
multiplied by 100%.
% by mass of an element = mass of element
mass of compound
x 100%
% by mass of element = mass of element in 1 mol compound
molar mass of compound
Practice
IN CLASS ONLY!
x 100%
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You can use percent composition to calculate
the number of grams of any element in a
specific mass of a compound
To do this, multiply the mass of the compound
by a conversion factor based on the percent
composition of the element in the compound
IN CLASS ONLY!
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Empirical formula of a compound gives the
lowest whole-number ratio of the atoms or
moles of the elements in a compound
The ratio of the number of moles of each
element in a compound gives the subscripts in
a compound’s empirical formula
How can you calculate the empirical formula of
a compound?
The percent composition of a compound can be
used to calculate the empirical formula of that
compound.
IN CLASS ONLY!
Key Question
How does the molecular formula of a compound
compare with the empirical formula ?
The molecular formula of a compound is either
the same as its experimentally determined
empirical formula, or it is a simple whole-number
multiple of its empirical formula.
Simple Whole-Number Multiple for Molecular Formulas
is the
molecular weight (molar mass of unknown)
Empirical formula weight
(molar mass of empirical formula)
*Fractions of atoms do not exist in compounds. In the case
where the ratio of atoms is a fractional number the ratio
should be simplified by multiplying all the atoms by a
constant to give whole number ratios for all the atoms
IN CLASS ONLY!