Transcript Slide 1

The Mole, Avogadro’s
Number & Molar Mass
Packet #20
COUNTING ATOMS BY
WEIGHING
Counting Atoms by Weighing I
• C(s) + O2(g)  CO2 (g)
• Suppose a scientist has a small
pile of carbon and wants to
know how many oxygen
molecules are required to
convert all of the carbon into
carbon dioxide.
• What can the scientist learn
from the chemical equation?
• The balanced equation
illustrates that one diatomic
oxygen molecule (O2) is
required for each carbon
atom.
Counting Atoms by Weighing II
• C(s) + O2(g)  CO2 (g)
• To determine how many
oxygen atoms are needed
by the scientist, the
scientist must know how
many carbon atoms are
present in his/her pile.
• To determine this, the
scientist must count the
number of atoms through
the weight of the sample
provided.
AMU to grams Conversion
• AMU
• Atomic mass unit
• 1 amu = 1.66 * 10-24 g
• The periodic table provides a
list of the average amu for each
atom
• Aka atomic mass
• Why?
• The atomic mass, listed on the
periodic table, is measured in
amu’s.
• These amu’s can be converted
into grams using the equation
provided.
Problem Example #1A
• Calculate the mass, in
amu, for 1000 neutral
carbon atoms
• (1000 atoms) * (12.01
amu/
atom)
• (1000 atoms) * (12.01
amu/
atom)
• = 12,010 amu
Problem Example 1B
• Let’s assume that when the scientist weighed the pile of
natural carbon, the amount weighed was 3.00 * 1020
amu. How many carbon atoms does the scientist have?
• Remember: -1 carbon atom = 12 amu
• Conversion factor
• Calculation
• 3.00 * 1020 amu * (1 carbon atom/12 amu)
• 3.00 * 1020 amu * (1 carbon atom/12 amu)
• 2.50 * 1019 carbon atoms
Problem Examples 2 - 5
• Calculate the mass, in amu, of a sample of aluminum
that contains 75 atoms.
• Calculate the mass, in amu, of a sample that contains 23
atoms.
• Calculate the number of sodium atoms present in a
sample that has a mass of 1172.49 amu.
• Calculate the number of oxygen atoms in a sample that
has a mass of 288 amu.
THE MOLE
Introduction
• A mole is a
counting unit
• Similar to a dozen
Mole
• Dozen = 12
• A mole is equal to 6.02
* 1023 items
• This is known as
Avogadro’s number
Counting
Unit
6.02 * 1023
atoms
Avogadro’s
number
Introduction II
• How many cars
(individual cars) are
found in 1 mole of cars
(group)?
• 1 mole of cars = 6.02 * 1023
cars
• How many cookies
(individual cookies) are
found in 1 mole of cookies
(group)?
• 1 mole of cookies = 6.02 *
1023 cookies.
Introduction III
• Example III
• One mole of carbon
(element)
• 1 mole C = 6.02 * 1023
atoms
• Remember
• An element is a
substance composed
of atoms that are all
the same.
MOLAR MASS
Molar Mass an of Atom
• The molar mass of
an atom is
equivalent to the
atom’s atomic
mass.
Molar Mass Example 1A
• What is the molar
mass of 1 mole of
oxygen (mono)?
• 16 grams
Molar Mass Example 1B
• What is the molar
mass of 3 mole of
oxygen?
• 3 * 16 grams
Molar Mass Example 1C
• How many moles are
found within 64
grams of oxygen?
• 4 moles
Molar Mass of a Molecule/Compound
• The molar mass of a
molecule/compound
is equivalent of sum
of the atomic masses
of the all the atoms
within the
molecule/compound.
Molar Mass of a Molecule/Compound II
• Calculate the molar
mass of one mole of
sulfur dioxide (SO2).
• 1 mole of sulfur =
32.07 g
• 1 mole of diatomic
oxygen (16.00 * 2 ) =
32 g
• 1 mole of SO2 = 64.07
g
Molar Mass of a Molecule/Compound II
• Calcium carbonate (calcite),
CaCO3 is the principal mineral
found in limestone, marble
chalk, pearls and the shells of
marine animals such as clams.
a. Calculate the molar mass of
calcite.
•
b.
100.09 g
A certain sample of calcite
contains 4.86 mol. What is
the mass in grams of this
sample?
•
486 g of CaCO3
Molar Mass of a Molecule/Compound III
• Juglone, a dye known for
centuries, is produced
form the husks of black
walnuts. It is also a
natural herbicide that
kills off competitive
plants around the black
walnut tree but does not
affect grass and other
non-competitive plants.
The molecular formula is
C10H6O3.
Molar Mass of a Molecule/Compound IV
• Calculate the molar mass
of juglone
• 174.1 g = molar mass of
juglone
• A sample of 1.56 g of
juglone was extracted
from black walnut husks.
How many moles of
juglone does this sample
represent?
• 8.96 * 10-3 mol juglone
Molar Mass of a Molecule/Compound V
• Isopentyl acetate, C7H14O2, the
compound responsible for the
scent of bananas, can be
produced commercially.
Interestingly, bees release
about 1 ug (1 * 10-6 g) of this
compound when they sting.
This attracts other bees, which
join the attack. How many
moles and how many
molecules of isopentyl
acetate are released in a
typical bee sting?
•
5 * 1015 molecules
REVIEW
Example #1
• How many amu are in 1 mole of carbon?
• 12 amu
• How many amu’s are in 2 moles of carbon?
• (12 amu / 1 mole )* (2 moles)
• (12 amu / 1 mole )* (2 moles) = 24 amu
• How many grams are in 2 moles of carbon?
• Already calculated: -2 moles of carbon = 24 amu
• 1 amu = 1.66 * 1024
• (1.66 * 10^24/1 amu ) * (24 amu)
• (1.66 * 10^24/1 amu ) * (24 amu) =