Avogadro’s constant and
the mole
• Mass on its own is not very useful
because when reactions take place
they depend on # of atoms not their
mass.
• Atoms are way to small and
numerous to count so you need a
way to relate # of atoms to masses
that can be measured
The Mole
• People find it easier and useful to
work in groups when counting
Pair=2 Dozen=12, Gross=144
• To count the number of atoms a
dozen or gross isn’t large enough
since elements contain a huge # of
atoms
• Instead chemist use the mole
(symbol mol)
The Mole
• The amount of substance that contains
6.02214199X1023 particles(atoms,
molecules, formula units) of that
substance
• This is called Avogadro Constant
• Has the symbol NA
• Rarely need 9 sig figs when working with
the number so 3 is enough
• NA=6.02X1023mol-1
Examples
• One mole of carbon contains
6.02X1023 carbon atoms
• One mole of Magnesium Chloride
contains 6.02X1023 formula units of
MgCl
• Etc
Application
• When someone ask you to buy them 2
dozen eggs, they are also thinking of 24
eggs
2dozen X 12eggs=24eggs
1dozen
• You can convert between moles and
particles in the same way
2molX6.02X1023atoms=1.20X1024atoms
mol
Converting moles to
# of particles
• Moles are used to help us count
atoms and molecules
# of moles
Multiply by
6.02X1023
# of particles
(atoms, molecules,
formula units)
• Relationship can be written using
formula below
Where: N=# of particles
n= amount(mols)
• N=n X NA
NA= Avogadro's
constant(mol-1)
Converting # of particles to
moles
• Usually express in moles
• To convert # of particles in a
substance to number of moles, just
rearrange the formula learned on
pervious slide
N= n X NA rearranges to
n= N/NA
Example
• How many moles are present in a sample of
potassium floride, KF, made up of 5.83X1024
molecules?
Solution: To solve we use Avogadro constant
and the number of molecules given and
substitute into equation:
n=
5.83X1024 molecules KF
6.02X1023 molecules KF/mol KF
n=9.68mol KF
So there are 9.68 moles of KF in the sample
Converting from moles to
mass
• Suppose you were asked to weigh out
three moles of a substance
• In order to do so you must figure out how
much 3 moles is in grams to use the scale
• The formula below relates moles and mass
by molar mass
• Mass=#of moles X molar mass
• m= n X M
where m=mass
n=# moles
M=molar mass
Example
• A flask contains 0.750 mol of carbon
dioxide gas. What mass of CO2 gas is in
this sample?
• First we must find the molar mass of CO2
to use in the equation along with the # of
moles that was given
• 1Carbon + 2Oxygen
12.01g/mol+2(16.00g/mol)=44.01g/mol
• Using the equation m=n X M
m= 0.750molX44.01g/mol
m=33g of CO2 in 0.750mol of CO2
Converting from mass to
moles
• Opposite of moles to mass
• Use formula below
• Mol= mass
or n= m
molar mass
M
Example
• How many moles of acetic acid, CH3COOH, are
there in a 23.6g sample?
• First need to find molar mass:
• There are 2C, 2O, and 4H
M=2(12.01g/mol)+(2(16g/mol)+4(1.01g/mol)
M=60.06g/mol
• Next substitute values into equation n=m/M
n= 23.6g
n= 0.393mol of CH3COOH in
60.06g/mol
23.6g of CH3COOH
Time to practice!