Chapter 7 Moles
What is a Representative Particle
• The smallest unit into which a substance
can be broken down without changing the
composition of the substance.
• Atoms, molecules, and formula units
What is a mole?
• A mole is a measurement
• Other measurements include:
– Couple, dozen, ream, gross, etc.
What is a mole (mol)?
1 mole = 6.02 x 1023
representative particles
• A HUGE NUMBER!
– 602,000,000,000,000,000,000,000
• Avogadro's Number
• Memorize this number!
Formulas
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Moles x 6.02 x 1023 = particles
Particle ÷ 6.02 x 1023 = moles
Moles x molar mass = grams
Grams ÷ molar mass = moles
Moles x 22.4 = liters
Liters ÷ 22.4 = moles
Mole Conversions—Moles to
Particles
• If you know the moles of a
substance, you can calculate the
number of particles in it.
• Use Avogadro’s number to
calculate
• Moles is the crossroad to all
conversions
Mole Conversions—Moles to
Particles
• Ex. How many water molecules
are there in 2.00 moles of water?
• 2 moles H20 x 6.02 x 1023 molecules
1.20 X 1024 H2O molecules
Problems
• How many moles of magnesium is 1.25 x
1023 atoms of magnesium?
• 1.25 x 1023 atoms Mg
( 6.02 X 1023 atoms Mg)
2.08 X 10-1 mol of Mg
or 0.208 mol Mg
• How many molecules are in 2.12 mols of propane
(C3H8)?
• 2.12 mol C3H8 x 6.02 x 1023 molecules
1.28 x 1024 molecules of C3H8
Think about this…
• If you had a mole of water, would you be
able to swim in it?
How can this happen?
• One mole of zinc is 65.4 grams
• One mole of calcium is 40.1 grams
• One mole of magnesium is 24.3 grams
• Different densities
Atomic Mass
• Protons and neutrons each have a mass of 1
atomic mass unit (amu)
• One mole of atomic mass units equals 1
gram.
• Carbon-12 has 12 amu
– One mole of carbon is 12 grams
• The mass of a mole of an element such as carbon
is called the gram atomic mass (gam)
• The mass of a mole of a covalent compound such
as water is called the gram molecular mass
(gmm)
• The mass of a mole of an ionic compound such as
NaCl is called the gram formula mass (gfm)
• All three of these can simply be called the
MOLAR MASS.
Finding the Molar Mass
• Find the molar mass of SO3
• Add the atomic masses of the atoms that
make the molecule. Round to the tenth.
• 1 S = 32.1
• 3 O: (3 x 16.0 amu) = 48.0
• 32.1 + 48.0 amu = 80.1 grams/mol
Mole Conversions—Mass to
Moles
• If you know the mass of a
substance, you can calculate the
number of moles.
Ex.: You have 11.2 g of NaCl. How
many moles is that?
First: Calculate the molar mass of NaCl.
Na = 23.0 g/mol
Cl = 35.5 g/mol
NaCl = 23.0 + 35.5 = 58.5 g/mol
This means 1 mol of NaCl = 58.5 g NaCl
Use the molar mass to calculate
the number of moles in 11.2 g of
NaCl.
Use the formula.
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11.2 g NaCl
58.5 g NaCl
Answer: 0.191 moles NaCl
Mole Conversions—Moles to
Mass
• If you know the number of
moles, you can calculate the
mass.
• Ex. How many grams are in
2.50 mol of NaCl?
• Use the molar mass formula to
convert to grams.
• 2.50 mol NaCl x 58.5 g NaCl
Answer: 146 grams of NaCl
Practice Problems
• How many moles are there in 1.50 x 1023
molecules NH3?
• Calculate the molar mass: N2O3 ?
• How many moles are in 15.5 g SiO2 ?
• What is the mass of 14.4 mol F2 ?
Practice Problems
• How many moles are there in 1.50 x 1023
molecules NH3? Answer: 0.249 mols NH3
• Calculate the molar mass: N2O3 ? Answer:
76.0 grams/mol
• How many moles are in 15.5 g SiO2 ?
Answer: 0.258 mols SiO2
• What is the mass of 14.4 mol F2 ?
Answer: 547 g F2
Volume of a Mole of Gas
• Volumes of gases vary with changes in their
temperatures and pressures
• In experiments, gas volumes are measured
at STP (standard temperature and pressure)
• Standard temperature is 0° C or 273 K
• Standard pressure is 1 atm or 101.3 kPa
• One mole of all gases at STP have a volume
of 22.4 L
Practice Problem
• What is the volume, at STP, of a balloon
containing 25.0 grams of carbon dioxide?
• 25.0 g CO2 = 0.568… mol CO2 x 22.4 L
44.0 g/mol
• 12.7 L CO2
Balanced Equations @ STP
• Coefficients are moles and molecules
• Moles x molar mass = grams
• Moles x 22.4 = L
CH4 + 2O2 CO2 + 2H2O @STP
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How many mole of H2O?
2 mol
How many molecules of H2O?
2 molecules
How many liters of O2?
2 x 22.4 = 44.8 L
How many grams of product?
12.0 + 32.0 + 4.0 + 32.0 = 80.0 g
Percent Composition
• The percent by mass of each element in a
compound
• % composition =
mass of element x 100
mass of compound
Practice Problem
• An 8.20 g piece of magnesium combines
with oxygen to form 13.60 g of magnesium
oxide. What is the percent composition of
this compound?
• % Mg = 8.20 g Mg ÷ 13.60 g MgO x 100
• 60.3 % Magnesium
• 13.60 g MgO – 8.20 g Mg = 5.40 g
• % O = 5.40 g O ÷ 13.60 g MgO x 100
• 39.7 % Oxygen
Practice Problem
• What is the percent composition of NaCl?
• Na is 23.0 g/mol + Cl is 35.5 g/mol = NaCl
at 58.5 g/mol
• % Na = 23.0 g/mol ÷ 58.5 g/mol x 100
• 39.3 % sodium
• % Cl = 35.5 g/mol ÷ 58.5 g/mol x 100
• 60.7 % chlorine
Problem Continued
• How much sodium is there in 454 g NaCl?
• 39.3 % ÷ 100 = 0.393
• 454 g NaCl x 0.393 = 178.422 = 178 g Na
Empirical Formula
• Shows the lowest whole-number ratio of the
atoms of the elements in a compound
• What is the empirical formula of C8H18?
• C4H9
• What is the empirical formula of H2O?
• H2O
Molecular Formula
• Shows the actual number and kinds of
atoms present in a molecule of a compound
• The empirical formula of a compound is
C2H5. The molar mass is 58.0 g/mol. What
is the molecular formula?
Molecular Formula Continued
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C = 12.0 g/mol and H = 1.0 g/mol
C2H5= 24.0 g/mol + 5.0 g/mol = 29.0 g/mol
58.0 g/mol ÷ 29.0 g/mol = 2
C2H5 x 2 = C4H10
Formula from Percent
• What is the formula for a compound of iron
and chlorine that is 34.4 % iron and 65.6 %
chlorine by weight?
• Assume 100 grams of compound. 34.4 %
becomes 34.4 g of Fe and 65.6% becomes
65.6 g of Cl
• Convert to moles.
34.4 g ÷ 55.8 g/mol = 0.656487.. mols
Continued
65.6 g ÷ 35.5 g/mol = 1.87887.. mols
Find the mole ratio (divide each mole value
by the smallest mole value)
0.656487.. mols ÷ 0.656487.. mols = 1
1.87887..mols ÷ 0.656487.. mols = 3.0
• The mole ratios are the subscripts.
FeCl3