3/5/09
Atomic
Masses
  The
atoms
of
different
elements
have
different
masses:
  Since
the
mass
of
an
atom
is
very
small,
we
use
a
special
unit
to
describe
it…
1
u
=
1/12
the
mass
of
C‐12
  For
Example:
  The
Magnesium
atom
has
12
protons,
12
neutrons,
and
12
electrons.
  Since
protons
and
neutrons
have
the
same
mass
of
approximately
1
u,
the
atomic
mass
at
Magnesium
is
24.3
u.
Try
some…
Molecular
Masses
  If
we
add
up
the
masses
of
ALL
the
atoms
that
make
up
a
molecule
or
a
formula
unit,
we
can
calculate
the
atoms
MOLECULAR
MASS.
  For
Example:
  Consider
NaCl:
1
x
Na
=
23.o
u
1
x
Cl
=
35.5
u
1
x
NaCl
=
58.5
u
Avogadro’s Number and!
the Mole!
When I buy eggs, how
many come in a
carton?
How many
doughnuts in a box?
How many cans of
pop in case? ∴12 = 1 dozen
1
3/5/09
  The
measure
of
a
mole
is
always
the
same:
  Because
chemists
deal
with
such
large
amounts
of
particles,
it
is
easier
to
work
with
moles
of
particles.
  What
makes
the
mole
useful
is
that
we
can
use
it
to
convert
between
the
mass
of
an
element
(in
grams)
and
the
number
of
atoms
present.
For
Example:
Could be written as 32.0 g/mol
How
big
is
a
mole?
1
mole
of
marshmallows
would
be
enough
marshmallows
to
make
a
19
km
thick
layer
of
marshmallows
covering
the
entire
face
of
the
Earth.
If
I
won
a
mole
of
dollars
in
the
lottery
that
would
be
equal
to:
$602,000,000,000,000,000,000,000.00
(602
sextillion
dollars)
1
mole
of
popcorn
kernels
could
be
spread
uniformly
over
the
USA
if
the
thickness
of
the
layer
was
about
14
km.
2
3/5/09
Homework:
Find
the
molar
mass
of
the
following:
  Do:
  W.S.
10‐1
  W.S.
4‐1
#1,
2
Using
the
Mole
DETERMINE
THE
MASS
OF
A
SUBSTANCE
GIVEN
THE
NUMBER
OF
MOLES
  Using
Avogadro’s
number
and
what
we’ve
learned
from
dimensional
analysis,
we
can
now
do..
MOLE
CONVERSIONS!!!!
For
Example:
  What
is
the
mass
in
grams
of
3.25
mol
of
CO2?
  How
many
moles
of
CH3OH
are
there
in
0.250
g
of
CH3OH?
CONVERSIONS
BETWEEN
NUMBERS
OF
PARTICLES
AND
MOLES
OF
PARTICLES
  You
must
determine
the
number
of
moles
that
are
present,
given
the
number
of
particles
present.
  Use
the
following
conversion
factors:
3
3/5/09
For
Example:
  How
many
moles
of
Carbon
are
there
in
3.01x1024
C
atoms?
CONVERSIONS
BETWEEN
NUMBER
OF
MOLES
AND
THE
VOLUME
OF
A
GAS
  Calculations
involving
gas
volumes
are
simplified
by
Avogadro’s
Hypothesis:
  How
many
molecules
does
3.65
moles
of
Nitrogen
dioxide
contain?
Equal
volumes
of
different
gasses,
at
the
same
temperature
and
pressure,
contain
the
same
number
of
particles.
STP
=
273
K
(0ºC
)
and
101.3
kPa
For
Example:
  What
is
the
volume
occupied
by
0.350
mol
of
SO2(g)
at
STP?
  How
many
moles
of
gas
are
contained
in
a
balloon
with
a
volume
of
10.0
L
at
STP?
Homework:
  Do:
  W.S.
4‐1
#3
  W.S.
10‐2
#1‐10,
12‐15,
23‐30
4
3/5/09
MULTI‐STEP
MOLE
CONVERSIONS
For
Example:
  Multi‐step
mole
conversions
require
the
use
of
two
or
more
conversion
factors.
  How
many
atoms
are
there
in
5
molecules
of
CuSO4•5H2O?
  Before
trying
multi‐step
conversions,
there
is
one
more
type
of
conversion
factor
to
be
aware
of:
  How
many
Hydrogen
atoms
are
there
in
30
molecules
of
H3PO4?
  Always
remember:
For
Example:
  What
is
the
volume
occupied
by
50.0
g
of
NH3(g)
at
STP?
  Problems
involving
density
require
you
to
recall:
  What
is
the
mass
of
1.00x1012
atoms
of
Cl?
  How
many
Oxygen
atoms
are
contained
in
75.0
L
of
SO3(g)
at
STP?
Where:
d
=
density
(in
g/mL
or
kg/L)
m
=
mass
(in
g
or
kg)
V =
volume
(in
mL
or
L)
5
3/5/09
For
Example:
Homework:
  What
is
the
volume
occupied
by
3.00
mol
of
ethanol,
CH3CH2OH(l)?
(d
=
0.790
g/mL)
  How
many
moles
of
Hg(l)
are
contained
in
100
mL
of
Hg(l)?
(d
=
13.6
g/mL)
  What
is
the
density
of
O2(g)
at
STP?
  Do:
 
 
 
 
W.S.
4‐1
#4‐12
W.S.
10‐2
#11,
16‐22
Multi‐Step
Mole
Conversion
W.S.
Study
for
Quiz
(in
two
classes).
Due
in
two
classes…
work
block
tomorrow.
  A
2.50
L
bulb
contains
4.91
g
of
a
gas
at
STP.
What
is
the
molar
mass
of
the
gas?
  Al2O3(s)
has
a
density
of
3.97
g/mL.
How
many
atoms
of
Al
are
in
100mL
of
Al2O3?
Percentage
Composition
For
Example:
  What
is
the
percentage
composition
of
CH4?
  What
is
the
percentage
composition
of
H2SO4?
  What
is
the
percentage
of
water
in
CuSO4
•
5H2O?
Empirical
Formula
For
Example:
  What
is
the
empirical
formula
of
a
compound
consisting
of
80.0%
C
and
20.0
%
H?
  A
compound
contains
58.8%
C,
7.3%
H,
and
34.1%
N.
What
is
the
empirical
formula
of
the
compound?
All
of
CH2,
C2H4,
C3H6,
C4H8,
and
C5H10
contain
twice
as
many
H’s
as
C’s
and
therefore
the
empirical
formula
for
all
these
molecules
is
CH2.
  What
is
the
empirical
formula
of
a
compound
containing
81.8%
C
and
18.2%
H?
  What
is
the
empirical
formula
of
a
compound
containing
39.0%
Si
and
61%
O?
6
3/5/09
Molecular
Formulae
  The
molecular
formula
gives
the
actual
number
of
atoms
of
each
element
in
a
molecular
compound.
  The
molecular
formula
can
be
found
by
dividing
the
molar
mass
of
the
compound
by
the
empirical
formula
mass.
Homework:
For
Example:
  Find
the
molecular
formula
for
a
compound
that
contains
4.90
g
N
and
11.2
g
O.
The
molar
mass
of
the
compound
is
92.0
g/mol.
  β‐carotene,
a
compound
found
in
carrots,
can
be
broken
down
to
form
vitamin
A.
The
empirical
formula
for
β‐carotene
is
C5H7.
The
molar
mass
of
β‐carotene
is
536
g/mol.
What
is
the
molecular
formula
for
β‐carotene?
Molar
Concentration
  Do:
  %
Composition
W.S.
  W.S.
10‐3
A couple of things to keep in mind…
7
3/5/09
Making
Up
Solutions
  Using
a
balance,
obtain
the
required
mass
of
solute
in
a
beaker.
  The
more
precise
the
balance
the
better.
  Dissolve
the
solid
in
distilled
water.
  Use
a
beaker
and
stirring
rod.
  Transfer
the
solution
and
rinse
all
equipment
into
a
clean
volumetric
flask.
  Rinse
at
least
three
times
to
transfer
all
solution.
  Use
a
stirring
rod
and
funnel
to
transfer
the
solution.
The
following
defines
molar
concentration:
  Add
distilled
water
up
to
the
calibration
line
on
the
flask.
An
eye
dropper
may
be
used.
  Avoid
parallax
error.
Look
directly
at
the
line.
  Stopper
the
volumetric
flask
and
mix
the
contents
thoroughly.
  Invert
and
shake
10
times,
holding
the
flask
closed.
For
Example:
  What
is
the
[NaCl]
in
a
solution
containing
5.12
g
of
NaCl
in
250.0
mL
of
solution?
  What
mass
of
NaOH
is
contained
in
3.50
L
of
0.200
M
NaOH?
Where:
C
=
molar
concentration
(mol/L)
n
=
#
of
moles
(mol)
V
=
volume
(L)
Homework:
  Do:
  Dilution
Calculations
W.S.
#1‐5
  What
is
the
molarity
of
pure
Sulfuric
acid,
H2SO4(aq),
having
a
density
of
1.839
g/mL?
  What
is
the
molarity
of
the
CaCl2
in
a
solution
made
by
dissolving
and
diluting
15.00
g
of
CaCl26H2O
to
500
mL?
Dilution
Calculations
A
simple
dilution
of
a
chemical
in
solution.
8
3/5/09
Mixing
two
solutions
having
different
concentrations
of
the
same
chemical.
For
Example:
  If
200.0
mL
of
0.500
M
NaCl
is
added
to
300.0
mL
of
water,
what
is
the
resulting
[NaCl]
in
the
mixture
  What
will
the
concentration
of
a
BaCl2
solution
be
if
23.0
mL
of
0.317
M
BaCl2
is
added
to
a
100.0
mL
volumetric
flask,
and
diluted
to
100.0
mL?
Making
dilute
solutions
from
concentrated
solutions.
For
Example:
  If
300.0
mL
of
0.250
M
NaCl
is
added
to
500.0
mL
of
0.100
M
NaCl,
what
is
the
resulting
[NaCl]
of
the
mixture?
  If
a
student
mixes
430.0
mL
of
0.100
M
BaCl2
with
125.0
mL
of
0.325
M
KI,
what
is
the
resulting
[KI]
in
the
final
mixture?
  The
procedure
used
is
similar
to
the
first
scenario,
but
now
we
are
solving
for
[…]old or Vinitial.
  This
requires
us
to
re‐arrange
the
equation
from
before
to
solve
for
the
unknown
variable.
For
Example:
  What
volume
of
6.00
M
HCl
is
used
in
making
up
2.00
L
of
0.125
M
HCl?
  What
volume
of
15.4
M
HNO3
is
needed
to
make
up
500.0
mL
of
0.100
M
HNO3?
Homework:
  Do:
  Dilution
Calculations
W.S.
#6‐11
  Read/Prep
Analysis
of
a
Hydrate
Lab.
  Study
for
Quiz.
9
3/5/09
What
is
on
the
exam?
  Introduction
to
the
Mole
  Atomic
Masses
  Molecular
Masses
  Avogadro’s
Number
and
the
Mole
  Using
the
Mole
  Single
and
Multi
Step
Mole
Conversions
  Know
your
conversion
factors!
  Density
Calculations
  Percent
Composition
  Empirical
and
Molecular
Formulae
  Finding
the
Empirical
formula
  Finding
the
Molecular
Formula
  Molar
Concentration
  Making
Up
Solutions
  Dilution
Calculations
10