Download The Mole

The Mole
 Just
to clear up any
misconceptions
when we use the
term “mole” we
are not referring to
this small blind
fellow
The Mole
 Instead
when we talk
about a mole in
chemistry we think of
this gentleman to the
right: Amedeo
Avogadro
The Mole
 So
what is the mole?
 In chemistry we have the need to count
how many atoms are in a particular
substance, yet if atoms are so small how
are we able to do this?
 Answer: We use a measurement called
the Mole
6.02x1023
 The
mole = 6.02x1023 representative particles the
same way that a dozen eggs = 12 eggs
Representative Particles
 Representative
particles are the species
present in a substance




Atoms (molecules for diatomic molecules)
Molecular compounds (molecules)
Ionic compounds (formula units)
One mole of any substance is always said
to contain 6.02x1023 representative particles
Representative Particles
 Examples



of representative particles:
One rep. particle of carbon dioxide, a
molecular compound, is made up of one
carbon atom and two oxygen atoms (CO2)
Helium is made up of helium atoms
Diatomic molecules such as hydrogen (H2)
contain two atoms of that particular
element (O2, N2, F2, Cl2 are all examples of
diatomic molecules)
Making the Connection
 We
can physically count how many eggs
are in one dozen, however it is almost
impossible, and extremely impractical, to
try and count how many rep. particles of
a compound (e.g. H2O) are in 1 mL of
solution.
 So, instead of counting we call that
number of particles a mole (mol) of that
compound.
Conversion Factors
 If
we are trying to determine how many
molecules are in one mole of a molecular
compound, such as CO2, we simply take
one mole of CO2 and use the conversion
factor 6.02x1023 molecules/1 mol of CO2
to determine how many molecules we will
have.
Conversion Factors
 If
we are trying to determine how many
atoms are in one mole of CO2 we must
perform the conversion we just did,
however, we must add another step:

How many atoms make up one molecule
of CO2?
Conversion Factors
 How
many atoms are in one mole of the
following compounds:


H2 O
NaCl
 How
many oxygen atoms are in 3 moles
of the following compounds:


Al2O3
C6H12O6
The Mass of a Mole of an
Element
 Let’s

first define the following term:
Molar Mass: the atomic mass of an element or
compound expressed in grams.
 Carbon
has an atomic mass of 12 amu
(atomic mass units) and, thus, a molar mass of
12 g.
 One mole of C is equal to the molar mass of
C.
 Similarly, we can say that one mole of any
element is equal to the element’s molar mass.
The Mass of a Mole of a
Compound
 How
do we determine the mass of one
mole of a compound?
The Mass of a Mole of a
Compound
 First,
we must look at the chemical
formula. For example: SO3, which is
called __________________.
 Second, we must determine the type and
number of atoms that make up the
compound. In this case SO3 consists of 1
sulfur atom and 3 oxygen atoms.
The Mass of a Mole of a
Compound
 Third,
we must determine the molar
mass of all of the atoms that make up
the compound and add them
together. For example:



1 S atom has a molar mass of 32.1g
3 O atoms have a molar mass of 3(16.0g)
For a total of 80.1g
 So,
we can say that the mass of one
mole of SO3 = 80.1g
The Mass of a Mole of a
Compound
 Calculating
molar mass values is an
important skill that you will use very often
in chemistry, especially when determining
how much of a compound that you will
need to add to a chemical reaction to
attain the desired product.
The Mass of a Mole of a
Compound
 Find




the molar mass of each compound:
C 2 H6
PCl3
C3H2OH
N2O5
Molar Mass
 Remember:
Molar mass can be defined
as the mass (in grams) of one mole of any
substance.
 The unit that we use to describe molar
mass is g/mol.
 Let’s look at an example:

How many grams are in 9.45 mol of
dinitrogen trioxide (N2O3)?
Molar Mass
 First,
we must map out our conversion
problem by looking at what we are
starting with and what we are looking for.

In this case we are going from moles of
N2O3 to grams of N2O3.
 Second,
we must determine the molar
mass of N2O3, which is?
Molar Mass
 Third,
we need to set up our linear
conversion (use the factor-label method
like we used with metric conversions)
using the molar mass as a conversion
factor:
 Last,
we obtain our answer, which is?
Moles to Mass
 Find
the mass, in grams, of each of the
following quantities:



3.32 mol K
4.52x10-3 mol C20H42
0.0112 mol K2CO3
Mass to Moles
 Find
the number of moles in each of the
following quantities:



0.370 g B
27.4 g TiO2
847 g (NH4)2CO3
Conclusion


We should now know how to convert from moles
to mass and from mass to moles.
Often times laboratory procedures describe
chemical reactions in which the amounts of
elements or compounds being added are
represented in moles. Yet if we are adding a solid
we must convert from moles to grams to add the
correct amount of reactant so that a successful
reaction takes place. Likewise, if we are adding a
liquid we must convert to units of volume (usually
mL) using density (D=M/V). We will go over how to
convert from moles of a gas to volume of a gas in
the rest of section 7.2.
Mole-Volume Relationships
 Review:
What are the units that describe
molar mass?
 What units do we use to describe the
volume of a mole?
Mole-Volume Relationships
 How
does temperature affect the volume
of a gas?
 How does pressure affect the volume of a
gas?
 http://phet.colorado.edu/simulations/sims
.php?sim=Gas_Properties
Mole-Volume Relationships
 Because
of the fact that the volume of a
gas is affected by temperature and
pressure, we must measure gasses at STP
(Standard Temperature and Pressure).


Standard temperature = 0ºC
Standard pressure = 1atm or 101.3 kPa
Mole-Volume Relationships
 At
STP one mole of any gas occupies a
volume of 22.4 L. This quantity is known as
the molar volume of a gas.
 We can also say that because 1 mol (of
gas) = 22.4L (of gas) and 1 mol = 6.02x1023
representative particles, then 22.4L of a
gas = 6.02x1023 representative particles of
that gas.
Moles to Liters
 What



is the volume at STP of these gases?
3.20x10-3 mol CO2
0.960 mol CH4
3.70 mol N2
Moles to Liters
 Assuming
STP, how many moles are in
these volumes?



67.2 L SO2
0.880 L He
1.00x103 L C2H6