UNIT 2
Moles and Molar Mass
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The Mole
represents a specific
amount of any
substance.
Specifically it
represents 6.02x1023
particles such as
atoms and
molecules.
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The mole is based
on the fact that 12
grams of Carbon-12
has a mole of
atoms.
A mole means you
have 6.02x1023
particles.
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Molar Mass (aka gram formula mass or
molecular mass) is the mass of one mole of a
substance.
Element’s molar masses are reported on the
periodic table.
1.
What is the molar mass of iron?
2.
What is the molar mass of copper?
55.8 g/mol
63.5 g/mol
1. What is the molar mass of water?
H2O = 2(1.0) + 16.0 =
18.0g/mol
2. What is the gram-formula-mass of calcium
chloride?
CaCl2 = 40.1 + 2(35.5) =
111.1 g/mol
Calculating Moles
Use the formula on table T:
number of moles = given mass (g)
_
gram-formula mass
(Given mass will be your answer.)
1.
Calculate the mass of 6.70 moles of carbon.
2.
Calculate the mass of 0.023moles of lithium.
3.
Calculate the mass of 25.02 moles of
calcium phosphate.
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Use the same formula to calculate the moles,
placing the number in the question on the
numerator:
number of moles = given mass (g)
gram-formula mass
4. Determine the number of moles in 8.0
grams of Boron.
5.
Determine the number of moles in 0.567
grams of Helium.
6.
Find the number of moles in 1230 grams of
magnesium sulfate.
Moles to Coefficients
4Al + 3O2  2Al2O3
reactants
products
Coefficients: How many moles of the substance
are needed in a reaction.
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To relate moles of one substance to another,
simply create a proportion:
4Al + 3O2  2Al2O3
If 3 moles of oxygen react, how many moles of
Aluminum oxide form?
2. If 4 moles of aluminum react, how many moles
of oxygen are needed?
3. If 4 moles of aluminum oxide are formed, how
many moles of oxygen were used?
4. If 8 moles of aluminum react, how many moles
of oxygen are needed?
1.
16 Al + 3S8  8Al2S3
1.
If 2.50 moles of sulfur react, how many
moles of aluminum sulfide form?
2.
If 3.75 moles of aluminum react, how many
moles of sulfur are needed?
Why do chemists use moles to measure
substances? Why aren’t grams, liters and
molecules enough?
Balancing
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In a reaction, atoms and molecules cannot
appear or disappear. Mass must stay constant
from the beginning to the end of the
reaction.
H2 + O2  H2O
___H2 + ___O2  ___ H2O
___ N2 + ____H2  ____ NH3
___Li + ____O2  ___Li2O
__Pb(NO3)2 +__K2CrO4___PbCr2O4 + ___KNO3
___C4H8 + ___O2  ___CO2 + ___H2O
1.
2.
3.
4.
5.
Synthesis:
Decomposition:
Combustion:
Single Replacement:
Double Replacement:
A + 2B  AB2
AB2  A + 2B
CH4 + O2  CO2 + H2O
AB + C  CB +A
AB + CD  AD + CB
* Notice synthesis and decomposition are
opposites. Also, combustion can have any
carbon compound as a reactant.
Empirical and Molecular Formulas
1.
2.
3.
4.
5.
Determine the mass of each element in water.
What is the ratio of mass of hydrogen to mass
of oxygen in water?
Is the ratio of mass related to the formula of
water?
Find the moles of H and O in water.
Is the ratio of moles related to the formula of
water?
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Empirical formula refers to any molecular
formula in it’s reduced form.
Are these empirical? If not, reduce them:
1.
2.
3.
4.
C 2H 2
C6H12O6
NO2
Na2(OH)2
1. CH
2. CH2O
3. NO2
4. NaOH
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Molecular Formulas are some multiple of the
empirical formula.
Example: If the empirical formula is CH4 a
molecular formula could be CH4, C2H8, C3H12
etc.
A compound whose empirical formula is NH3
has a mass of 34 g/mol. What is the empirical
formula?
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Find the mass of the empirical formula.
Divide the mass given by the empirical mass.
Distribute your answer through the empirical
formula.
If a compound has a mass of 45 g/mol and an
empirical formula of CH3, what is the
molecular formula?
Percent Composition
Nutrition Facts on foods
can tell you just how
much of a substance you
are consuming and how
that relates to how much
you should eat in a day.
 It is equally important to
know how much of a
element or compound is
in a mixture.
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By mass: mass part x 100
total mass
1.
Find the % by mass of phosphoric acid.
2.
Find the percent by mass of Ca in Ca(OH)2.
NaClO
2. NaCl
3. NaOH
1.
23.0/74.5 *100 = 30.9%
23.0/58.5 *100 = 39.3.%
23.0/40.0 *100 = 57.5%
1. A sample of a substance containing
only magnesium and chlorine was
tested in the laboratory and found to be
composed of 74.5% chlorine by mass. If
the total mass of the sample was 190.2
grams, what is the mass of the
magnesium?