Plan for Fri, 26 Sept 08
• Diagnostic Quiz returned
– Average = 64.7 +/- 18.1%
• Lecture
–
–
–
–
–
Naming acids and their anions (2.8)
Counting by weighing (3.1)
More on isotopes and the amu (3.2)
Meet the Mole, your new best friend (3.3)
Molar mass (3.4)
• Also read:
– 3.5 (% composition of compounds); 3.6 (determining the formula
of a compound)
• Quiz 1 at the end of class
What’s an acid?
• Certain compounds, when dissolved in water,
lose H+ ions to the water molecules, producing
the H3O+ (hydronium) cation and an anion:
H2O + HA
H3O+ + A-
• These compound are called acids.
• The anion “A” may be monatomic or polyatomic.
Naming Acids
• The acids we will consider have the general
form:
HnXOm
where n 1, and m
0
• The name of the acid depends on the name of
the element X and the value of m.
• The name of the anion produced from acid
ionization depends on the name of the acid and
the value of n.
Wow, that’s
a lot of and
rules! their
If only we
could
Naming
Acids
Anions
see them generalized in some sort of table…
1.
Find the –ic acid…your reference point for number of oxygens. Anion is
–ate. You will have to memorize these.
HClO3 … chlor-ic acid  ClO3- … chlor-ate ion
2.
Acids with one fewer O than the –ic are –ous acids. Anion is –ite.
HClO2 … chlor-ous acid  ClO2- … chlor-ite ion
3.
Acids with one more O than the –ic are per–ic acids. Anion is per–ate.
HClO4 … per-chlor-ic acid  ClO4- … per-chlor-ate ion
4.
Acids with two fewer O’s than the –ic are hypo–ous acids. Anion is
hypo–ite.
HClO … hypo-chlor-ous acid  ClO- … hypo-chlor-ite ion
5.
Acids with no O’s compared to the –ic are hydro–ic acids. Anion is –ide.
HCl … hydro-chlor-ic acid ClO3- … chlor-ide ion
The Bob (Bo) Acids
Bo Acid
Acid Name
HBoOm+1
per-bob-ic
HBoOm
.
HBoOm-1
bob-ic
bob-ous
HBoOm-2 hypo-bob-ous
HBo
hydro-bob-ic
Bo Anion Anion Name
BoOm+1
BoOm
per-bob-ate
.
bob-ate
BoOm-1
bob-ite
BoOm-2
hypo-bob-ite
Bo
bob-ide
Examples
• Given bromic acid, HBrO3,
–
–
–
–
–
Name BrO3- … bromate ion
Name HBrO2 … bromous acid
Write hypobromous acid … HBrO
Write perbromate ion … BrO4Name HBr … hydrobromic acid
• Given acetic acid, CH3COOH,
– Name CH3COO- … acetate ion
• Given phosphorous acid, H3PO3,
– Write phosphoric acid … H3PO4
The –ic Acids You Should Know
Ions from
total acid
ionization
are –ate ions
Total acid
ionization is
often just a
theoretical
limit.
What about the number of H’s?
• We said before that the name of the anion depends on
the name of the acid and the number of H’s.
• On the last slide we learned that anions from total acid
ionization (all H’s removed) are the –ate ions.
• For polyprotic (many H’s) acids, more than one H
removal is possible:
H3PO4 + H2O
H2PO4- + H3O+
H2PO4- + H2O
HPO42- + H3O+
• Naming these acid anions is similar to naming
compounds of two nonmetals:
dihydrogen phosphate
hydrogen phosphate
Ch 3 - Stoichiometry
• Today
– Isotopes and average atomic mass (3.2)
– Counting by weighing (3.1)
– The Mole (3.3)
– Molar Mass (3.4)
• Monday
– Balancing chemical equations (3.8)
– Stoichiometric Calculations (3.9)
– Limiting Reagent (3.10)
Chemical Reactions are “Recipes”
•
A recipe specifies that the following ingredients…
–
–
–
–
–
–
–
1 ¾ cup cake flour
3 tsp baking powder
2 oz. baking chocolate
1 ½ cup sugar
½ cup butter
4 eggs
½ cup milk
…are necessary to bake one chocolate cake.
•
Similarly, chemical reactions include information about how much of each
reactant you need to produce a certain amount of product.
CH4 + 2O2
•
•

CO2 + 2H2O
Chemistry occurs on the atomic/molecular level, but it is extremely difficult
to keep track of one atom or molecule at a time.
We need a way to relate a macroscopic measurement, like grams, to
the number of atoms or molecules in a sample.
Isotopes and Atomic Mass
• Atoms of the same element that differ in mass
(eg. 12C, 13C, 14C)
– isotopes are the same element
– isotopes have the same number of protons
– isotopes differ in the number of neutrons, and therefore
atomic mass (which is measured in amu).
• Many isotopes occur in nature. Most natural
isotopes are not radioactive, nor are they
necessarily harmful.
• A sample of an element will contain some
percentage of all its isotopes.
Determining isotopic mass using a
Mass Spectrometer
• Stream of vaporized atoms is bombarded with
high-speed electrons, which knock electrons off
the gaseous atoms, turning them into cations.
• Gaseous cations are accelerated through
magnetic field, and their paths are bent according
to their mass.
Calculating Avg Atomic Mass
Let’s say we have a sample of 1000 carbon (C) atoms. Based on
isotopic abundance:
989 weigh 12 amu (98.9%)
11 weigh 13 amu (1.1%)
What is the average mass of a carbon atom in this sample?
(989 C atoms)*(12 amu) + (11 C atoms)*(13 amu)
Avg. mass =
1000 C atoms
(989 C atoms)*(12 amu) + (11 C atoms)*(13 amu)
=
1000 C atoms
1000 C atoms
=
(989/1000)*(12 amu) + (11/1000)*(13 amu)
=
(0.989)*(12 amu) + (0.011)*(13 amu)
=
12.011 amu
Some Isotope Comparisons
Mass of Isotope
(amu)
Relative
Abundance
Average Atomic
Mass (amu)
Hydrogen
1.00782
2.01410
99.9844%
0.0156%
1.0079
Carbon
12 (exact)
13.00335
98.892%
1.108%
12.01115
Sulfur
31.972071
32.971458
33.967867
35.967080
95.06%
0.74%
4.18%
0.0136%
32.064
Lithium
6.015123
7.016005
7.5%
92.5%
6.941
Element
A Brief History of the amu
• Stanislao Canizzaro (1826-1910) proposed that the H atom be used
as a standard of mass and set its atomic mass at 2.
• Other chemists of the day wanted to use a more massive atom to
reduce experimental error.
• Chemists eventually took the mass of naturally occurring oxygen (O)
to be 16 amu.
• Concurrently, physicists defined the oxygen-16 isotope as 16 amu.
Are these two definitions that use oxygen the same?
NO. Naturally occurring oxygen is a combination of three
isotopes, including oxygen-16.
These two definitions resulted in conflicting values.
• Finally in the 1950s the carbon-12 isotope was adapted
as the standard…hence 1 amu = 1/12 the mass of one
12C atom.
Don’t confuse “Atomic Mass” with
the mass of one atom!!
• An atom can be only one isotope at a time.
•
•
12C:
Z = 12, atomic mass = 12 amu (exactly)
13C: Z = 13, atomic mass = 13.003354 amu
• The Atomic Mass (or Average Atomic Mass) is the
average of the atomic masses of all the chemical
element's isotopes, weighted by isotopic abundance.
• Naturally occurring carbon has atomic mass of 12.011
amu
• There is no carbon isotope that weighs 12.011 amu.
How many jelly beans are in this jar?
We know the following…
• mass of the jar + beans = 2 kg
• mass of the empty jar = 0.5 kg
• the average mass of a jelly
bean = 1 g
• Mass of just the beans = 1.5 kg
? Jelly beans = 1500 g
1 jelly bean
1g
= 1500 jelly beans
How many 12C atoms in 12.0 g?
Remember the jellybeans!!
Mass of one carbon-12 atom: 1.992 646 632 x 10-23 g
Divide 12.000 000 00 g by the mass of a single 12C atom:
12.000 000 00 g of carbon
1.992 646 632 x 10-23 g/C atom
=
6.022 141 511 x 1023 C atoms
THE
MOLE, a
chemical
“dozen.”
The Mole: a chemical “dozen”
The mole is the amount of substance that contains as
many elementary entities as there are carbon atoms in
0.012 kg of carbon-12. Its symbol is “mol.”
Note: the elementary entities must be specified and may
be atoms, molecules, ions, electrons, other particles, or
specified groups of such particles.
Avogadro’s Number (NA)
_ 0.000 000 30) x 1023 mol-1 (entities/mole)
(6.022 141 79 +
How big is 6.022 x 1023?
•
NA pennies stacked on top of one another would produce a
stack 7.1 x 1018 miles tall, tall enough to reach from the earth to
the sun and back almost 75 billion times!
~93 million miles
•
If NA pennies were distributed equally among all 5 billion people
on Earth, every single earthling would have 1.2 trillion dollars!
•
So…given that there are 6.02 x 1023 (1 mol) C atoms in about 1
tbsp soot, about how small is an atom?
The period at the end of this sentence could hold about 1017
(1,000,000,000,000,000,000) atoms.
•
Dang!
What does one mole look like?
For condensed-phase
substances (solids and
liquids), one mole is a
convenient “hand-full” sized
quantity.
1 mol N2(g),
V = 22.4 L
For gas-phase substances
at room temperature at sea
level, one mole has a
volume of about 22.4 L.
1 mol NaCl(s)
1 mol H2O(l)
Aluminum (Al)
atomic weight = 26.98 amu.
molar mass = 26.98 g/mol
1 mol Al contains
6.022 x 1023 Al atoms
Lead (Pb)
atomic weight = 207.2 amu.
molar mass = 207.2 g/mol
1 mol Pb contains
6.022 x 1023 Pb atoms
water
O
H
H
1 mol sucrose
contains 6.022 x 1023
sucrose molecules.
1 mol water contains
6.022 x 1023 water
molecules.
HO
H 2O
Molc.Wt.: 18.02 amu
MM: 18.02 g/mol
C
H
CH3
HO
C 3 H 8O
Molc.Wt.: 60.10 amu
MM: 60.10 g/mol
CH
H2C
1 mol isopropyl
alcohol contains
6.022 x 1023
isopropyl alcohol
molecules.
H2
C
HO
HO
O
C
O
CH
CH
CH
CH
CH
OH
CH
CH
sucrose (table sugar) O
isopropyl alcohol
CH3
HO
CH2
OH
OH
C12H22O11
OH Molc.Wt.: 342.3 amu
MM: 342.3 g/mol
Table salt (NaCl).
formula mass = 58.44 amu.
molar mass = 58.44 g/mol
1 mol of table salt contains 6.022
x 1023 NaCl formula units…this
means:
6.022 x 1023 sodium ions (Na+)
and…
6.022 x 1023 chloride ions (Cl-)
Interpreting the Mole
How much does one mole of 1-amu particles weigh?
Recall: 1 amu = 1/12 the mass of one carbon-12 atom
= 1.660 538 86 x 10-24 g/amu
? g/mol =
6.022 x 1023 amu
1 mol
1.660 x 10-24 g
1 amu
= 0.9999992 g/mol
~ 1 g/mol
This shows how the mole concept relates the microscopic
mass system (atomic mass unit or amu) to the macroscopic
(kilogram) system. It gives us a conversion factor between
the two systems:
1 g = NA amu
How can we use 1 g = NA amu?
• Let’s find the molar mass of carbon-12 (how much a
mole of 12C atoms weighs):
6.022 x 1023 12C atoms
12 amu
1g
1 mol 12C
1 12C atom
NA amu
? g/mol =
= 12 g/mol
• What if we had 1 mol of naturally occurring carbon, a
mixture of isotopes?
6.022 x 1023 C atoms
? g/mol =
1 mol carbon
= 12.011 g/mol
12.011 amu
avg mass of 1 C atom
1g
NA amu
What is the main
assumption we are making
in this calculation?
1500 g
1 jelly bean
= 1500 jelly beans
1g
Basically, we are pretending
that all the jellybeans are
identical, and all weigh 1 g.
But really, the masses of
jellybeans in a sample will
vary somewhat.
We treat a sample of atoms the same
way…we treat them as though they were
identical and had a mass equal to the
average atomic mass.
Molar Mass
• The mass in grams of one mole of a substance.
• The mole is defined in such a manner that the atomic
weights given in the periodic table can be interpreted as
molar masses.
For example, find the molar mass of CO2.
molar mass of O
2*(molar mass of O)
molar mass of C
=
=
=
+
15.999 g/mol
31.998 g/mol
12.011 g/mol
44.009 g/mol
Example
• What is the molar mass of acetic acid, CH3COOH?
molar mass of O =
molar mass of C =
molar mass of H =
15.999 g/mol
12.011 g/mol
1.0079 g/mol
2*(molar mass of O)
2*(molar mass of C)
4*(molar mass of H)
=
=
=
31.998
24.022
+ 4.0316
60.0516
g/mol
g/mol
g/mol
g/mol
60.052 g/mol