#### Transcript Sec. 3.4 - Morrison Community Unit District 6

```Wednesday, Nov. 6th: “A” Day
Thursday, Nov. 7th: “B” Day (11:45 release)
Agenda
 Collect “Introduction to the Elements” Worksheet
 Section 3.4: “Counting Atoms”
Atomic mass unit, atomic mass, mole, molar
 Homework:
Sec. 3.4 review, pg. 104: #1-10
Concept Review: “Counting Atoms”
 Mole Project/Mole Movie
*Quiz next time over section 3.4*
Section 3.4: “Counting Atoms”
You wouldn’t expect something as small as an
atom to have much mass.
For example, copper atoms have an average
mass of 1.0552 X 10-25 Kg.
Since atoms are so small, kilograms or even
grams are not convenient units to use for their
masses.
This is where the atomic mass unit comes into
play…
Atomic Mass Unit
Scientists created a new unit, called the atomic
mass unit, or amu.
Atomic mass unit: 1/12 the atomic mass of the
C-12 isotope.
Protons and neutrons each have a mass of about
1 amu.
So, an atom’s mass number (from the periodic
table) is equal to the atom’s mass in amu’s.
mass # = mass in amu’s
(remember, mass # = protons + neutrons)
Masses of Atoms are Expressed in
Atomic Mass Units (amu’s)
Atomic mass: the mass of an atom expressed
in atomic mass units (amu’s)
How can you tell the atomic mass of an
element?
Check the periodic table: the mass number
shown on the periodic table is an average of
the atomic masses of the naturally occurring
isotopes of that element
Introduction to the Mole
Words that stand for numbers:
Pair: 2
Dozen: 12
Gross: 144
Ream: 500
Mole:
1 Mole =
602,200,000,000,000,000,000,000
(6.022 X 10 23)
Mole
Most samples of elements contain huge numbers
of atoms.
To make working with these numbers easier,
chemists created a new unit call the mole (mol)
Mole: the number of atoms in exactly 12 grams of
carbon-12.
(Remember, the mole is the SI base unit for the
amount of a substance.)
Avogadro’s Number: 6.022 X 1023, the number of
particles, atoms, or molecules in 1 mole of a
substance.
1 mole = 6.022 X 1023 particles, atoms, or
molecules
6.022 x 1023 is called
Just How Big is a Mole?
A mole of marbles stacked over the
United States would cover the country
to a depth of 70 miles!
Just How Big is a Mole?
If 1 mole of dollars were equally distributed
to every person in the world, about 7
billion people, each person would receive…
\$86,000,000,000,000
How do we Measure Moles?
Calculations in chemistry require moles, but
can you go to one of the balances in our lab
and measure the number of moles?
NO!
That’s where the molar mass comes into
play…
Molar Mass
To convert between moles and grams,
chemists use the molar mass of a substance.
Molar mass: the mass, in grams, of 1 mole of
a substance.
Units: g/mol (grams per mole)
The molar mass of an element is equal to the
element’s mass number found on the periodic
table.
Molar Mass
What is the molar mass of iron?
55.85 g/mol
What is the molar mass of rubidium?
85.47 g/mol
What is the molar mass of gold?
196.97 g/mol
What is the molar mass of silver?
107.87 g/mol
What is the molar mass of calcium?
40.08 g/mol
Molar Mass
How do you find the molar mass of a
compound, like water?
Simply add up the atomic masses of the
elements that make up the compound.
Water, H2O, consists of 2 hydrogen atoms and
1 oxygen atom:
H: 1.01 g/mol
H: 1.01 g/mol
O: 16.00 g/mol
18.02 g/mol
Molar Mass
What is the molar mass of salt, NaCl?
Na: 22.99 g/mol
Cl: 35.45 g/mol
58.44 g/mol
What is the molar mass of sodium hydroxide,
NaOH?
Na: 22.99 g/mol
O: 16.00 g/mol
H: 1.01 g/mol
40.00 g/mol
‘A Mole in One’
1 mole = 6.022x1023 atoms = an atom’s mass # in grams
Converting from Moles
Mass
Sample Problem D, pg. 102
What is the mass, in grams, of 3.50 mole of
copper, Cu?
Use the periodic table to find the molar mass
of copper, Cu.
The conversion factor needed to solve the
problem is:
1 mole Cu = 63.55 g Cu
3.50 mole Cu X 63.55 g Cu =
1 mole Cu
222 g Cu
(3 sig figs)
Practice Problem #3, pg 102
Calculate the number of moles of 0.850 g of
hydrogen atoms. What is the mass in grams
of 0.850 mol of hydrogen atoms?
Use the periodic table to find the molar mass
of hydrogen, H.
The conversion factor needed to solve the
problem is: 1 mole H = 1.008 g H
0.850 g H X 1 mol H = .843 mol H
1.008 g H
0.850 mol H X 1.008 g H = .857 g H
1 mol H
(3 sig figs each)
Converting from Moles
# of Atoms
Sample Problem E, pg. 103
Determine the number of atoms in 0.30 mol of
fluorine atoms.
The conversion factor needed to solve the
problem is:
1 mole F = 6.022 X 10 23 atoms F
0.30 mol F atoms X 6.022 X 1023 atoms F
1 mole F
1.8 X 1023 atoms F
(2 sig figs)
Practice Problem #2, pg. 103
How many moles of silver are represented by
2.888 X 1023 atoms?
The conversion factor needed to solve the
problem is:
1 mole Ag = 6.022 X 10 23 atoms Ag
2.888 X 1023 atoms X 1 mole Ag
6.022 X 1023 atoms
.4796 mol Ag
(4 sig figs)
Homework
(you may work with a partner)
Section 3.4 review
pg 104: 1-10
Concept Review: “Counting Atoms”