THE MOLE CONCEPT
The mole, an SI unit abbreviated “mol”, is used
to measure the amount of matter present in a
sample. One mole contains as many particles
(atoms, molecules, or other fundamental unit) as
there are atoms in exactly 12 grams of the
isotope carbon-12.
1 mole = 6.0221367 x 1023 particles
What are the properties of these particles?
All matter has a mass and occupies space.
The mass of one mole of matter has many
names but the most useful are the molecular
mass (or weight) and the formula mass (or
weight).
The volume of one mole of any ideal gas is 22.4
liters at STP.
FORMULA MASS
To find the formula mass of one mole of a
substance, you must find the sum of the masses
of all the atoms listed in the formula. For
example,
Determine the formula mass for carbon dioxide,
CO2.
one mole of carbon dioxide, CO2, contains
What would be the mass of one mole of water,
H2O?
one mole of water, H2O, contains
Find the mass of molecular mass for glucose,
C6H12O6.
One mole of glucose, C6H12O6, contains
When writing formulas, parenthesis are used to
show there is more than one ion containing
more than one element.
Determine the formula mass for ammonium
sulfate, (NH4)2SO4.
one mole of (NH4)2SO4 contains
DENSITY
The density of a substance is the ratio of the
mass of an object to its volume. Since all matter
has both a mass and a volume, the density is a
physical property useful for identifying
substances.
The units
of density
are usually
grams per cubic centimeter, g/cc or g/cm3, for
liquids and solids, and grams per liter, g/R, for
gases.
The density of a gas is the mass of one liter at
STP.
50.0 grams of a gas occupies 20.0 liters at STP.
Calculate the density of the gas.
Proportion method:
? grams = one liter
50.0 grams = 20.0 liters
“Plug-nChug”
method:
mass =
50.0 volume
grams =
20.0 R
Fi
n
d
the mass of 40.0 liters of nitrogen, N2, at STP.
(The density of nitrogen is 1.25 g/l, Chart C)
? grams = 40.0 liters
1.25 grams = one liter
V=
40.0
liters
D=
1.25 g/R
M = 1.25 g/R x 40.0 R = 50.0 grams
Calculate the volume occupied by 10.0 grams of
air at STP. (Density of air = 1.29 g/l at STP, from
Chart C)
10.0 grams = ? liters
1.29 grams = one liter
M=
10.0
grams
D=
1.29 g/R
Calculate the density of oxygen, O2, at STP.
1 mole = formula mass = 22.4 R at STP
Formula mass = 2 x O = 2 x 16 = 32 g.
? grams = one liter
32.0 grams = 22.4 liters
mass =
32.0
grams
volume
= 22.4
R
Calculate the density of carbon monoxide, CO, at
STP?
1 mole = formula mass = 22.4 R at STP
Formula mass = C(12 g) + O(16 g) = 28 g
? grams = one liter
28.0 grams = 22.4 liters
mass =
28.0
grams
volume
= 22.4
R
D
ifl
u
oromethane gas has a density of 2.32 g/R at STP.
What is the molecular mass of difluoromethane?
1 mole = molecular mass = 22.4 R at STP
? grams = 22.4 liters
2.32 grams = one liter
Dens
ity =
2.32 g/R
volume = 22.4 liters
Mass = 2.32 g/R x 22.4 R = 52 grams at STP
What is the mass of 33.6 liters of argon, Ar,
measured at STP?
Since Argon, Ar, has an atomic mass of 40 g.
1 mole = formula mass = 22.4 R at STP
? grams = 33.6 liters
40 grams = 22.4 liters
Determine the volume occupied by 256 grams of
sulfur dioxide, SO2, at STP.
1 mole = formula mass = 22.4 R at STP
Formula mass = 1S (32 g) + 2O(2 x 16)
1 mole = formula mass = 64 grams
256 grams = ? liters
64 grams = 22.4 liters
Calcul
ate the
mass
of 16.8
liters
of
propan
e,
C3H8,
at
STP?
1 mole = formula mass = 22.4 R at STP
Formula mass = 3C (12 g) + 8H(8 x 1g)
1 mole = formula mass = 44 grams
? grams = 16.8 liters
44 grams = 22.4 liters
Calcul
ate the
densit
y of
neon
gas,
Ne, at
STP.
1 mole
= atomic mass = 22.4 R at STP
atomic mass = 20 grams
? grams = one liter
20.0 grams = 22.4 liters
Determine the volume occupied by 300. grams of
freon-12, CCl2F2, at STP.
1 mole = formula mass = 22.4 R at STP
Formula mass = 1C + 2Cl + 2F =
= 1 x 12 g + 2 x 35 g + 2 x 19 g = 120 g
1 mole = formula mass = 120 grams
300. grams = ? liters
120 grams = 22.4 liters
A certain gas has a density of 2.68 g/R at STP.
What is the molecular mass of this substance?
1 mole = molecular mass = 22.4 R at STP
? grams = 22.4 liters
2.68 grams = one liter
Dens
ity =
2.68 g/R
volume = 22.4 liters
Mass = 2.68 g/R x 22.4 R = 60. grams at STP
Using the actual density of the gas as a guide,
which of the following substances is least ideal?
Compound
Actual Density at
STP
methane, CH4
0.716 g/R
oxygen, O2
1.43 g/R
sulfur dioxide, SO2
2.92 g/R
xenon, Xe
5.85 g/R