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Moles &
Conversions
2.3, 3.3, & 9.1
Atomic Mass
&
Formula Mass
Atomic Mass

is the mass of one atom and
is listed on the periodic table
with units of amu.
Atomic Mass
Samples: Masses:
H
 Li
O
F
 Al






Formula Mass

is the sum of the atomic
masses of all atoms in a
compound with units of amu.
Formula Mass
Samples: Masses:
 H2O

 LiOH

 OF2

 AlF3

Mole,
Avagadro’s Number
&
Molar Mass
Mole

In chemistry, we need a way to
count atoms and other small
particles. We use something
called the mole for this. It is a
counting number like:
1 pair = 2
 1 dozen = 12

Mole

1 mole = 6.022 x 1023
particles (particles may be
atoms, molecules, or formula
units)
Avagadro’s Number

To find the number of
particles in a mole, scientists
used the number of atoms in
12 g of C-12 as a standard.
In that sample, there are
6.022 x 1023 C-12 atoms.
Avagadro’s Number

That number of C-12 atoms
in 12 g is called Avagadro’s
number.
Avagadro’s Number

Avagadro’s number is
defined as the number of
particles in exactly one mole
of a pure substance which is
always 6.022 x 1023.
What’s in a mole?
6.022 x 1023 atoms per mole (of
atoms)
 6.022 x 1023 molecules per mole (of
molecules)
 6.022 x 1023 formula units per mole
(of formula units)

What’s in a mole?
More than 6.022 x 1023 atoms: 1 mole
CCl4 molecules is (5 × 6.022 x 1023)
atoms
 More than 6.022 x 1023 atoms: 1 mole
NaCl formula units is (2 × 6.022 x
23
10 ) atoms.

Molar Mass

is the mass of one mole of
a compound with units of
grams.
Molar Mass

The atomic mass on the
periodic table can also be
used for the molar mass
of an element - switch
amu with g.
Molar Mass
1 mole H atoms = _ atoms = _ g
 1 mole N atoms = _ atoms = _ g
 1 mole S atoms = _ atoms = _ g

Molar Mass

Why are there different
masses?
atoms have different masses just
like 12 eggs have different mass
than 12 grapefruit
 this is due to the varying
numbers of protons, neutrons, &
electrons

Mole Conversions
will use dimensional analysis to
solve problems.
 allow us to convert between moles,
particles (atoms, molecules, etc.),
mass, and volume.
 use
the molar masses and/or
volumes for solving problems.

Mole Conversions


If there are 16 g of S, how many
moles are there?
If there are 2 moles of N atoms,
what would the mass be?
Mole Conversions


Find the mass in grams of 3.50
moles of Cu.
If you have 11.9 g of Al, how
many moles do you have?
Mole Conversions


How many moles of Ag are in
3.01 x 1023 atoms?
What is the mass in grams of
1.20 x 108 atoms of Cu?
Mole Conversions
Molar volume
 the volume of 1 mole of gas
at STP equals 22.4 liters
Mole Conversions


A chemical reaction makes 0.82
moles of O2 gas. What is the
volume at STP?
A room has a volume of 4000. L.
How many moles of air would
that be (STP)?
Mole Conversions

A 1.0 L flask of CO2 (@ STP)
contains how many CO2 molecules?
Percent
Composition
7.3
Percent Composition
Percent Composition

is the % by mass of an element in a
compound
 molar mass gives 100% of compounds
mass because it accounts for each element
present in the compound
 to get % composition, take [(mass of the
element)/(molar mass of compound)] *
100%
Percent Composition
Percent Composition
Examples…
 Find the percent composition of
Cu2S.

Find the percent composition of
PbCl2.
Percent Composition
Percent Composition
Examples…
 Find the percent composition of
Ba(NO3)2.

Find the percent composition of
H in (NH4)2CO3.
Percent Composition
Percent Composition
Examples…
 Magnesium hydroxide is
54.87% O by mass. How many
grams of O would be in 175 g
of the compound?
Percent Composition
Percent Composition
Examples…
 Magnesium hydroxide is
54.87% O by mass. How many
grams of O would be in 175 g
of the compound? How many
moles of O is that?
Empirical &
Molecular
Formulas
7.4
Empirical Formulas
Empirical Formulas

consist
of
the
elements’
symbols and the subscripts that
show the smallest wholenumber ratio of the atoms
Empirical Formulas
Finding empirical formulas:



Convert the % composition data to
grams. (HINT: If not given grams, use
100 grams of compound to start.)
Convert the composition in grams to
moles by using molar masses of the
elements. (This gives a mole ratio.)
Use the numbers from mole ratio to get
the smallest possible whole number
ratio.
Determining
Empirical Formulas
Ex 1) A compound has 63.52%
Fe and 36.48% S. Find the
simplest formula.
 1. Comp. by mass:
 2. Comp. in moles:
 3. Smallest whole # ratio:
 4. Simplest formula:
Determining
Empirical Formulas
Ex 2) A 10.150 g substance sample
contains only P & O. After analysis,
it is determined that there is 5.717
g O. What is the simplest formula
of the compound?




1. Comp. by mass:
2. Comp. in moles:
3. Smallest whole # ratio:
4. Simplest formula:
Determining
Empirical Formulas
Ex 3) A compound has 26.56%
potassium, 35.41% chromium, and
38.03% oxygen. Find the simplest
formula for the compound.
 1. Comp. by mass:
 2. Comp. in moles:
 3. Smallest whole # ratio:
 4. Simplest formula:
Determining
Empirical Formulas
Ex 4) A compound has 53.70%
Fe and 46.30% S. Find the
simplest formula.
 1. Comp. by mass:
 2. Comp. in moles:
 3. Smallest whole # ratio:
 4. Simplest formula :
Molecular Formulas

We’ve now seen how to
calculate empirical formulas,
but that’s not always going to
give the actual formula of a
compound. (ex: AlCl3, Al2Cl6,
Al3Cl9, etc…)
Determining
Molecular Formulas

What we can determine is that
there is a relationship between
the simplest (empirical) formula
and molecular formula.
 It is (simplest formula)
x =
molecular formula.
 x is a whole number
Determining
Molecular Formulas

To determine the correct
formula, we need the formula
mass. This will be related to
the molecular formula mass in
the same way as the formulas
were related.
 (simplest formula mass)
x
= molec. formula mass
Determining
Molecular Formulas
Ex 1) The simplest formula of a
compound was determined to
be P2O5. The formula mass was
determined to be 283.889 amu.
What is the molecular formula?
 Find simplest mass:
 Find mass relationship:
 Find formula relationship:
Determining
Molecular Formulas
Ex 2) Determine the molecular
formula of a compound with an
empirical formula of CH and a
formula mass of 78.110 amu.
 Find simplest mass:
 Find mass relationship:
 Find formula relationship:
Determining
Molecular Formulas
Ex 3) A compound with formula mass of
42.08 amu is found to have a
composition by mass of 85.64% C and
14.36% H. Find the molecular formula.







Comp. by mass:
Comp. in moles:
Smallest whole # ratio:
Simplest formula:
Find simplest mass:
Find mass relationship:
Find formula relationship: