Chemical Quantities
Chapter Overview
Chemical Quantities
measuring matter
mass
count
volume
length
mole
gram
atomic mass
gram
formula mass
gram
molecular mass
molar
volume
Measuring Matter
• Mass
• ________
• Counting
• ________
• Volume
• _____________
• Length
• ________
Counting but not really
counting
If you know the mass of ______ item
you can find out how many items there
would be in a greater quantity.
How many apples are there in
11.0 kg?
Given
1 dozen apples = 2kg (mass)
Step 1
11.0 kg X conversion =
factor
How many apples are there in
11.0 kg?
Given
1 dozen apples = 2kg (mass)
Step 1
11.0 kg X _ dozen apples
_ kg
____ dozen apples
=
Step 2
5.5 dozen apples * 12 apples
=
1 dozen apples
Answer
• ____ apples
This method is counting but not
really counting because it is an
approximation of the answer.
Stoichiometry
The study of relationships that
can be derived from chemical
formulas and equations.
Quick Time™a nd a
GIF dec ompr esso r
ar e nee ded to see this pictur e.
Formula Mass
Sum of the ________ ________
formula mass
• if given in ________ or ______
molecular mass
• if substance is molecular
molar mass
• if given in ______
also called
gram atomic mass
gram formula mass
THE MOLE
• Number of particles
– like the problem with the apples, particles
are also represented in this way except
they are called moles instead of dozens.
• Avogadro’s number
–________
– this number is the unit particles per mole
• unit referring to molecules, atoms, and
formula units
Molar Volume
• ___ _____by one mole of particles
–If it is a solid or liquid find the mass
of one mole and divide by _______
–If it is a gas:
• One mole occupies _______ at STP
»STP = 0 °C and 101.3 kPa
Problems involving moles
Calculating molar mass
_______ the masses of each of the
separate elements in the formula
• Example
–What is the molar mass of
H2O?
EXAMPLE FIND THE MOLAR
MASS OF WATER
• Step One
–first find the molar mass of hydrogen.
–Each hydrogen atom contains 1.0 gram
per mole. Multiply one by two because
there are two hydrogen atoms.
• Answer to step one:
2.0 grams
Step Two
– find the mass of the oxygen
molecules.
–The mass of one oxygen is 15.99.
But you can round up to 16.
–There is only one oxygen molecule
so multiply 16 by one.
–Answer to step two:
• 16 grams
• Step Three
– add the masses of the oxygen
and the hydrogen together.
–16 g + 2g = _______ g
–Therefore the molar mass of
H2O is
• _______
grams per mole.
“Moles” is central
÷
Mass
Moles
X
Molar mass
Convert mass to moles
• To convert mass to moles you
need to ______ the mass by the
molar mass.
Example
How many moles are in
4.0 grams of H2O?
How many moles are in
4.0 grams of H2O?
• Step one
–find the molar mass of the formula
–as in the previous example take 1.0
times 2 for the hydrogen and 16.0 for
the oxygen.
–Add them together to get 18.0 grams
Step 2
•take the 4 grams of H2O and
_______ them by the molar mass
of water to convert mass into
moles.
• The math you would use for step
2 is as follows:
• 4.0g H2O X 1 mole H2O = . ___ mole
18 g H2O
This answer can also be written as
_________ mole
Converting
moles to mass
• Converting moles to mass is
similar to converting mass to
moles because you are taking
the equation backward
• To convert, multiply mass by
molar mass.
Example: How many grams
are in 4 moles of H2O?
• Step One
–find the molar mass of the
formula.
–From previous examples we
know that the molar mass of
H2O is 18 grams.
• Step Two
–once you find the 18 grams
for the molar mass of water,
take the 4 moles of water
and _______ by the molar
mass.
• The math equation for step two is shown
below:
• 4 moles H2O X 18 grams H2O =
1mole H2O
• The answer is ____ grams.
• Notice in the equation above the moles cancel
out to leave the grams for the answer.
Convert Molecules To Moles
• To convert _______ to moles
divide the number of molecules
by Avogadro’s number
(_______).
• One mole of any substance has
the same number of molecules
per mole
• Example
–How many moles are in
1.806 X 1024 molecules of water?
Take the number of molecules
and divide by Avogadro’s
number
• The math equations would look as
follows:
• 1.806X1024 atoms X 1 mole water =
_______
The answer :
_______ moles of water.
Once again notice that the atoms cancel each
other out.
“Moles” is central
÷
Mass
Moles
X
Molar mass
÷
X
Avogadro’s
number
Molecules
Converting Moles to
Molecules
• To convert moles to molecules
_______ the moles by Avogadro’s
number (6.02X1023)
Converting Moles to Molecules
• Example
–How many molecules are in
173 moles of H2O?
• Step One
–Take the number of moles and
_______ by Avogadro’s number
• The math equation would look as
follows:
• 173mole X 6.02X1023molecules =
1 mole
The answer is:
____________
molecules
Gas Volume at STP to Moles
• STP = 0°C and 101.3 kPa
• To convert gas volume to moles
–_______ the number of Liters by
22.4L/mole (Molar volume of any
gas at STP)
EXAMPLE
HOW MANY MOLES ARE IN 34.6 L
OF H2O(g)?
Volume ÷ molar volume = moles
• 34.6 L ÷ 22.4 L/mole
= 1.54 mole
At STP
“Moles” is central
Volume, gas
÷
Mass
÷
X Molar volume
Moles
X
Molar mass
÷
X
Avogadro’s
number
Molecules
Moles to Gas Volume
• To convert moles to gas
volume you have to take the
number of moles and
_______ it by 22.4 L/mole if
conditions are at STP.
EXAMPLE
How many Liters are there in 73.6
moles of H2O(g) ?
• Take 73.6 moles X 22.4Liters/mole
• _______ Liters at STP
Combination Calculations Example
• How many molecules are in 29.43
Liters of oxygen gas at STP?
•29.43 L ÷ ______Liters/mole =
•29.43 L X 1 mole = ______mole
22.4 L
•1.31 mole X 6.02 X 10 23 molecules =
1 mole
•____________ molecules
Mass of one atom
• Divide the molar mass by Avogadro’s
number
Mass of one atom
• What is the mass of one oxygen atom?
16.0 grams ÷ 6.02 X 10 23 atoms
mole
1 mole
16.0 grams X 1 mole__________ =
mole
6.02 X 10 23 atom
__________ g/atom
Gas Density
• The density of a gas is usually
measured in grams per LITER
• It can be used to calculate the
molar mass of a gas.
Molar mass from gas density
• Density = mass / volume
• If the mass you’re looking for is
Molar Mass then use the Molar
Volume
• The molar volume of any gas at
STP is 22.4 Liters / mole
Molar mass from gas density
• What is the molar mass of CO2 if its density
at STP is 1.96 g / L ?
• D = Molar mass
Molar volume
• 1.96 g / L = Molar mass
22.4 L / mole
• Molar mass = 1.96 g X 22.4 L
L
mole
• Molar mass
= _____g / mole
Percent Composition by Mass
• Percent composition is the relative
amounts of the elements in the
compound.
• To calculate the percent composition,
take the ___of the required ____
and divide by the _______ of the
compound.
Percent Composition Example
• Calculate the percent composition of
propane, C3H8.
• First get the total mass of the compound
• 3 moles C X 12 g/mole = 36 grams C
• 8 moles H X 1 g/mole = 8 grams H
• Total mass of propane
= 44 grams
Percent Composition C3H8
• %C = 36.0 grams C
X 100
44.0 grams propane
= _______ %
• %H = 8.0 grams H
44.0 grams propane
= _______ %
X 100
Determining Empirical Formula
• The empirical formula shows the
lowest whole number of _______
in a compound.
– For example the empirical formula of
benzene, C6H6 is CH.
A 1 : 1 ratio
Determining Empirical Formula
• To calculate the empirical formula first
get the number of ____of each element
• Then get the lowest ____ ____ ____
of those moles.
– These numbers become the ________
• If data is given as percent assume a
100 gram sample and just change the
percent to grams.
Example Empirical Formula
• What is the empirical formula
of a substance that is 80.0% C
and 20.0% H by mass?
–First change % to grams by
assuming a 100 g sample
• 80.0 g C and 20.0 g H
Empirical Formula Example
• Convert grams to moles (÷molar mass)
• 80.0 grams ÷ 12.0 grams/mole C
• = 6.67 moles C
• 20.0 grams ÷ 1.01 grams/mole H
• = 19.8 moles H
• The ____ of moles of H to C is
calculated
19.8 moles / 6.67 moles = 2.97 or 3 : 1
Empirical Formula
• Therefore the Empirical formula of
the compound is CH3
Molecular Formula
• The molecular formula shows the
____ ____ of ____ atom in the
molecule.
– For example: C6H6 shows ____
atoms of carbon and ____ atoms of
hydrogen in the benzene molecule
Molecular Formula
• To calculate the molecular formula from
the empirical formula
– First find the molar mass of the empirical
formula
– Next find the molar mass of the molecule
– Then ____ the molar mass of the molecule
by the ____ ____ of the empirical formula
to find how many times ____ the molecule
is than the empirical formula.
If the molar mass of ethane is 30.0 grams
per mole and the empirical formula as
determined in a previous example is CH3,
what is the molecular formula of ethane?
• First: The molar mass of CH3 is
12.0 + 3(1.01) = 15.0 g/mole
• Second: The molar mass of ethane is
30.0 g/mole
• Third: The ratio of molecule : empirical
is 30.0 g/mole ÷ 15.0 g/mole
= ____
So the molecular formula is ____
times ____ than the empirical
formula.
2 X CH3 = C2 H6
CREDITS
• Produced by
–Jay Seymour
–Patricia Brown
–1999