CHEMICAL QUANTITIES
Composition Stoichiometry
Calculating Molar Mass
Avogadro’s Number and the Mole
Percentage Composition and Empirical Formulas
Molecular Formulas
Reaction Stoichiometry
THE MOLE
The mole (mol) is the SI unit for the
amount of substance that contains as
many particles as there are in exactly 12
g of carbon-12.
AVOGADRO’S NUMBER
A mole of any substance contains
6.02×1023 representative particles of that
substance.
This is known as Avogadro’s Number.
REPRESENTATIVE PARTICLES
A representative particle is the smallest
particle of a substance that has all of the
properties of that substance.
REPRESENTATIVE PARTICLES
Different substances are made up of
different types of representative particles.
For example:
• Elements are made up of atoms.
• Molecular compounds and diatomic
elements are made up of molecules.
• Ionic compounds are made up of formula
units.
AVOGADRO’S NUMBER
1 mole = 6.02×1023 atoms
(monatomic element)
1 mole = 6.02×1023 molecules
(molecular compound or diatomic molecule)
1 mole = 6.02×1023 formula units
(ionic compound)
MOLAR MASS
The molar mass of a substance
is the mass in grams of one mole
of that substance.
The units for molar mass are
g/mol.
•
MOLAR MASS OF AN ELEMENT
The molar mass of an element is
numerically equal to the atomic mass of an
element in atomic mass units (which can
be found on the periodic table).
What is the molar mass of lithium?
6.94 g/mol
What is the molar mass of carbon?
12.01 g/mol
MOLAR MASS OF A COMPOUND
OR A DIATOMIC ELEMENT
The molar mass of a compound or a
diatomic element is calculated by adding
together the masses of the elements in a
mole of the molecules or formula units that
make up the substance.
MOLAR MASS CALCULATIONS
What is the molar mass of oxygen
gas?
O2 = 2 ×16.00 = 32.00 g/mol
MOLAR MASS CALCULATIONS
What is the molar mass of water?
H = 2×1.01 g/mol = 2.02 g/mol
O = 1×16.00 g/mol = 16.00 g/mol
2.02 g/mol + 16.00 g/mol = 18.02 g/mol
MOLAR MASS CALCULATIONS
What is the molar mass of calcium
nitrate?
Ca2+, NO3- =
Ca(NO3)2
Ca = 1×40.08 g/mol = 40.08 g/mol
N = 2×14.01 g/mol = 28.02 g/mol
O = 6×16.00 g/mol = 96.00 g/mol
40.08 g/mol + 28.02 g/mol + 96.00 g/mol=
164.10 g/mol
CALCULATIONS INVOLVING MOLES
The heart of chemistry is the mole.
Representative
Particles
Mass (g)
(atoms, molecules
or formula units)
moles
Volume (L)
MOLAR VOLUME
STP stands for standard temperature and
pressure.
Standard temperature is 0°C.
Standard pressure is 1 atm.
A mole of any gas at STP occupies a
volume of 22.4 L.
This is known as the molar volume of a gas.
1. Calculate the number of moles
of NaCl in 175.5 g of the salt.
First determine the molar mass of NaCl.
23.00 g/mol + 35.45 g/mol = 58.45 g/mol
= 3.003 mol NaCl
2. Calculate the mass of
0.800 mol of H2SO4.
First determine the molar mass of H2SO4.
2.02 g/mol + 32.06 g/mol + 64.00 g/mol = 98.09
g/mol
= 78.5 g H2SO4
3. How many sodium atoms
are in 2.5 mol of sodium
metal?
= 1.5×1024 atoms Na
4. A sample of nitrogen gas contains
1.20×1025 molecules. Determine
the number of grams of nitrogen
in the sample.
=599 g N2
5. How many atoms of calcium
are in a 10.0 g sample of
calcium metal?
= 1.50×1023 atoms Ca
6. Determine the volume, in
liters, of 0.600 mol of
sulfur dioxide gas at STP.
7.
Determine the number
of molecules in 33.6 L
of hydrogen gas at
STP.
= 9.03×1023 molecules H2
8.
Determine the mass of
carbon dioxide that occupies
a volume of 2.5 L at STP.
= 4.9 g CO2
PERCENTAGE COMPOSITION
Percentage composition is the percent
by mass of each element in a
compound.
FIND THE PERCENTAGE COMPOSITION OF
ALUMINUM SULFATE.
First determine the chemical formula for aluminum sulfate.
Al3+, SO42- Al2(SO4)3
Next determine the mass of each element present in the
compound and the molar mass of the compound.
Finally, determine the percent by mass of each element
present in the compound.
FIND THE PERCENT BY MASS OF
CARBON IN CARBON DIOXIDE.
First determine the chemical formula for carbon dioxide.
Carbon Dioxide = CO2
Next determine the mass of each element present in
the compound and the molar mass of the compound.
C = 1×12.01 = 12.01 g/mol
O = 2×16.00 = 32.00 g/mol
12.01 g/mol + 32.00 g/mol = 44.01 g/mol
Finally, determine the percent of carbon.
Hydrates
A hydrate is an ionic compound which
contains water within its crystal structure.
Anhydrous salt
Water of hydration
BaCl2•2H2O
Hydrate
HYDRATES
A hydrate can be heated to remove the
water.
The salt that is left after the water of
hydration has been removed is called the
anhydrous salt.
•
CALCULATING THE PERCENT BY MASS
OF WATER IN A HYDRATE
The percentage by mass of water in a
hydrate can be calculated using the
following formula:
Find the percent by mass of water in
MgSO4•7H2O.
First determine the molar mass of the water of
hydration.
7(2.02 + 16.00) = 126.14 g
Next determine the molar mass of the
anhydrous salt.
24.31 g + 32.07 g + 64.00 g = 120.38 g
Finally, calculate the percent of water.
A student heats 25.0 g of a hydrated compound to determine
the mass of water in the hydrate. After heating the resulting
anhydrous salt has a mass of 18.0 g. What is the percent by
mass of water in the hydrated compound?
EMPIRICAL FORMULAS
An empirical formula consists of the symbols
for the elements combined in a compound,
with subscripts showing the lowest wholenumber ratio of the different atoms in the
compound.
Examples: CH4, NH3, CO, LiClO3
Why aren’t N2H4 or C2H6 examples of
empirical formulas?
They are not in the smallest whole-number
ratio.
STEPS FOR DETERMINING THE EMPIRICAL FORMULA
1.
Determine moles of each element
2.
Divide by the smallest value. If each resulting number is
a whole number then they represent the subscripts.
3.
If the numbers obtained in the previous step are not
whole numbers, multiply each number by an integer in
order to obtain a whole number.
1. What is the empirical formula of a compound
that is 79.8% carbon and 20.2% hydrogen?
First determine the number of moles of each
element.
CH3
Next divide each answer by the smallest
number of moles.
2. What is the empirical formula of a compound that is
25.9% nitrogen and 74.1% oxygen?
N2O5
3. A compound is found to contain 46.0 g sodium,
52.0 g chromium, and 64.0 g oxygen. What is the
empirical formula of the compound?
Na2CrO4
4. A sample of a hydrated compound has a mass of 170.0 grams.
after drying the sample, a mass of 95.3 grams is recorded.
determine the formula for the hydrate, LINO3•xH2O.
First determine the mass of water driven off.
Next determine the moles of anhydrous salt and water.
Finally, determine the value of x.
LiNO3•3H2O
MOLECULAR FORMULAS
Nonmetallic elements can bond together in more
than one way. For example, carbon and oxygen
can form CO and CO2.
The molecular formula of a compound indicates
the types and numbers of atoms in a single
molecule of a molecular compound.
MOLECULAR FORMULAS
A molecular formula can be the same as the
experimentally determined empirical formula, or it
can be a whole-number multiple of it.
For example, the molecular formula for methane is
CH4. This is also the empirical formula because it
is in the lowest whole-number ratio.
The molecular formula for hydrogen peroxide is
H2O2. The empirical formula for hydrogen
peroxide is not the same as the molecular formula.
The empirical formula would be HO.
CLASSIFY EACH OF THE FOLLOWING
CHEMICAL FORMULAS AS EMPIRICAL,
MOLECULAR OR BOTH. JUSTIFY YOUR
ANSWER.
LiCl
Empirical Formula only
Ionic Compound
NH3
Both, Smallest ratio and molecular
compound
C2H2 Molecular Formula Only, It is a
molecular compound but it is not in the
smallest ratio
Which of the following are possible
molecular formulas for CH4?
C2H8
C4H12
C5H20
C6H24
C2H8, C5H20 and C6H24 are
possible molecular formulas.
In order to determine the
molecular formula of a
compound, you must know the
following information:
1.
2.
Determine the empirical
formula
ratio of the molar mass of the
molecular formula to the molar
mass of the empirical formula
1. Determine the molecular formula of CH4N.
The molar mass is 120.0 g/mol.
CH4N = 30.0 g/mol
The molecular formula is C4H16N4.
2. The molar mass of a compound is 92 g/mol.
analysis of a sample of the compound indicates
that it contains 0.606 g N and 1.390 g O. Find its
molecular formula.
We must first find the empirical formula.
The empirical formula is NO2. NO2 = 46 g/mol.
The molecular formula is N2O4.
3. The percent composition of a compound is
40.0%C, 6.7% H, and 53.3% O. The molar mass
of the compound is 90.0 g/mol. What is the
molecular formula of the compound?
Once again, we must first find the empirical formula.
The empirical formula is CH2O.
CH2O = 30.0 g/mol
The molecular formula
is C3H6O3.