Sec. 3.4 - Morrison Community Unit District 6
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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
mass, Avogadro’s number, conversions
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
Decade: 10
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
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
“Avogadro’s Number”
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”
Keep updating your KWL!
Remember: Quiz next time
over section 3.4
Mole Project
Your project is to create a mole to share with
the class.
The project will be done outside of regular
class time.
The projects will be due and presented to the
class on:
“A” Day: Wednesday, Nov. 20th
“B” Day: Thursday, Nov. 21st