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 rules! If only we could see them generalized in some sort of table…Naming Acids and their Anions

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 Bo Anion Anion Name

HBoOm+1 per-bob-ic BoOm+1 per-bob-ate

HBoOm . bob-ic BoOm . bob-ate

HBoOm-1 bob-ous BoOm-1 bob-ite

HBoOm-2 hypo-bob-ous BoOm-2 hypo-bob-ite

HBo hydro-bob-ic Bo bob-ide

Examples• Given bromic acid, HBrO3,

– Name BrO3- … bromate ion

– Name HBrO2 … bromous acid– Write hypobromous acid … HBrO– Write perbromate ion … BrO4

-

– Name HBr … hydrobromic acid

• Given acetic acid, CH3COOH,– Name CH3COO- … acetate ion

• Given phosphorous acid, H3PO3,– Write phosphoric acid … H3PO4

The –ic Acids You Should KnowIons 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 HPO4

2- + H3O+

• Naming these acid anions is similar to naming compounds of two nonmetals:

dihydrogen phosphatehydrogen 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 MassLet’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?

Avg. mass = (989 C atoms)*(12 amu) + (11 C atoms)*(13 amu)

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

ElementMass of Isotope

(amu)Relative

AbundanceAverage 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

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 g1 jelly bean

1 g= 1500 jelly beans

How many 12C atoms in 12.0 g?

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.”

Remember the jellybeans!!

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)

(6.022 141 79  0.000 000 30) x 1023 mol-1 (entities/mole)+_

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!

• 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.

~93 million miles

Dang!

What does one mole look like?

1 mol N2(g), V = 22.4 L

1 mol H2O(l)

1 mol NaCl(s)

For condensed-phase substances (solids and liquids), one mole is a convenient “hand-full” sized quantity.

For gas-phase substances at room temperature at sea level, one mole has a volume of about 22.4 L.

Lead (Pb)

atomic weight = 207.2 amu.

molar mass = 207.2 g/mol

1 mol Pb contains 6.022 x 1023 Pb atoms

Aluminum (Al)

atomic weight = 26.98 amu.

molar mass = 26.98 g/mol

1 mol Al contains 6.022 x 1023 Al atoms

OH

CH2C

CHOHCH

HO

CH

O

H2C

HO

O

CH

CH

OHCH

OH

CH

HO

CH

H2C

HO

O

sucrose (table sugar)

C12H22O11 Molc.Wt.: 342.3 amu

MM: 342.3 g/mol

H

O

H

water

H2OMolc.Wt.: 18.02 amu

MM: 18.02 g/mol

C

CH3

CH3

HO

isopropyl alcohol

H

C3H8OMolc.Wt.: 60.10 amu

MM: 60.10 g/mol

1 mol water contains 6.022 x 1023 water

molecules.

1 mol isopropyl alcohol contains

6.022 x 1023 isopropyl alcohol

molecules.

1 mol sucrose contains 6.022 x 1023 sucrose molecules.

1 mol of table salt contains 6.022 x 1023 NaCl formula units…this means:

Table salt (NaCl).

formula mass = 58.44 amu.molar mass = 58.44 g/mol

6.022 x 1023 sodium ions (Na+)

6.022 x 1023 chloride ions (Cl-)

and…

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:

Interpreting the Mole

Recall: 1 amu = 1/12 the mass of one carbon-12 atom = 1.660 538 86 x 10-24 g/amu

1 g = NA amu

? g/mol = 6.022 x 1023 amu 1.660 x 10-24 g

1 amu1 mol= 0.9999992 g/mol

~ 1 g/mol

How much does one mole of 1-amu particles weigh?

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):

• What if we had 1 mol of naturally occurring carbon, a mixture of isotopes?

? g/mol =

= 12 g/mol

12 amu6.022 x 1023 12C atoms

1 mol 12C 1 12C atom

1 g

NA amu

? g/mol =

= 12.011 g/mol

12.011 amu6.022 x 1023 C atoms

1 mol carbon avg mass of 1 C atom

1 g

NA amu

What is the main assumption we are making

in this calculation?

1500 g1 jelly bean

1 g= 1500 jelly beans

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 = 15.999 g/mol2*(molar mass of O) = 31.998 g/molmolar mass of C = 12.011 g/mol

44.009 g/mol

+

Example• What is the molar mass of acetic acid, CH3COOH?

molar mass of O = 15.999 g/molmolar mass of C = 12.011 g/mol molar mass of H = 1.0079 g/mol

2*(molar mass of O) = 31.998 g/mol2*(molar mass of C) = 24.022 g/mol4*(molar mass of H) = 4.0316 g/mol

60.0516 g/mol

60.052 g/mol

+