Download Chapter 4 The Mole Concept and Atoms

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Chapter 4
18 February 2014
Calculations and the Chemical
Equation
The Mole Concept and Atoms
• Atoms are exceedingly small
– Unit of measurement for mass of an atom is
atomic mass unit (amu) – unit of measure for
the mass of atoms
• carbon-12 assigned the mass of exactly 12 amu
• 1 amu = 1.66 x 10-24 g
• Periodic table gives atomic weights in amu
1
Mass of Atoms
• What is the atomic weight of one atom of
fluorine? Answer: 19.00 amu
• What would be the mass of this one atom
in grams?
19.00 amu F 1.66110-24 g 3.156 10 23 g F


F atom
1 amu F
F atom
• Chemists usually work with much larger
quantities
– It is more convenient to work with grams
than amu when using larger quantities
The Mole and Avogadro’s Number
• A practical unit for defining a collection
of atoms is the mole
1 mole of atoms = 6.022 x 1023 atoms
• This value is called Avogadro’s number
– This has provided the basis for the concept
of the mole
Amedeo Avogadro 1776-1856
2
The Mole
• To make this connection we must define the
mole as a counting unit
– The mole has symbol mol
• A mole is simply a unit that defines an amount
of something
–
–
–
–
–
dozen means 12
gross means 144
score means 20
ream means 500
mole means 6.022x1023
Atomic Mass
• The atomic mass of one atom of an element
corresponds to:
– The average mass of a single atom in amu
– The mass of a mole of atoms in grams
– 1 atom of F is 19.00 amu
19.00 amu/atom F
– 1 mole of F is 19.00 g 19.00 g/mol F
19.00 amu F 1.66 10 24 g F 6.022 10 23 atom F


1 atom F
1 amu F
1 mol F
=19.00 g F/mol F or 19.00 g/mol F
3
19.00 amu F 1.66 10 24 g F 6.022 10 23 atom F


1 atom F
1 amu F
1 mol F
=19.00 g F/mol F or 19.00 g/mol F
note:
The name of the unit is “mole”
The symbol is “mol”
Calculating atoms, moles, and mass
• We use the following conversion factors:
• Density converts grams – milliliters
• Atomic mass unit converts amu – grams
• Avogadro’s number converts moles –
number of atoms
• Molar mass converts grams – moles
4
Strategy for Calculations
• Map out a pattern for the required conversion
• Given a number of grams and asked for
number of atoms
• Two conversions are required
• Convert grams to moles
1 mol S/32.06 g S OR 32.06 g S/1 mol S
• Convert moles to atoms
mol S x (6.022 x 1023 atoms S) / 1 mol S
Practice Calculations
1. Calculate the number of atoms in 1.7 moles
of boron. (1.7 x 6.022x1023)
2. Find the mass in grams of 2.5 mol Na
(sodium). (2.5 x 23)
3. Calculate the number of atoms in 5.0 g
aluminum. ( (5.0/27) x 6.022x1023)
4. Calculate the mass of 5,000,000 atoms of
Au (gold). ( (5,000,000/6.022x1023) x 197)
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Interconversion Between Moles,
Particles, and Grams
The Chemical Formula, Formula
Weight, and Molar Mass
• Chemical formula - a combination of
symbols of the various elements that make up
the compound
• Formula unit - the smallest collection of
atoms that provide two important pieces of
information
– The identity of the atoms
– The relative number of each type of atom
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Chemical Formula
Consider the following formulas:
• H2 – 2 atoms of hydrogen are chemically bonded
forming diatomic hydrogen, subscript 2
• H2O – 2 atoms of hydrogen and 1 atom of oxygen,
lack of subscript means one atom
• NaCl – 1 atom each of sodium and chlorine
• Ca(OH)2 – 1 atom of calcium and 2 atoms each of
oxygen and hydrogen, subscript outside parentheses
applies to all atoms inside
Chemical Formula
Consider the following formulas:
• (NH4)2SO4 – 2 ammonium ions and 1 sulfate ion
– Ammonium ion contains 1 nitrogen and 4 hydrogen
– Sulfate ion contains 1 sulfur and 4 oxygen
– Compound contains 2 N, 8 H, 1 S, and 4 O
• CuSO4.5H2O
– This is an example of a hydrate - compounds containing
one or more water molecules as an integral part of their
structure
– 5 units of water with 1 CuSO4 (pentahydrate)
Homework: What does “sesqui” mean?
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Comparison of Hydrated and
Anhydrous Copper Sulfate
Hydrated copper sulfate
Anhydrous copper sulfate
Marked color difference illustrates the fact
that these are different compounds
Formula Weight and Molar Mass
• Formula weight - the sum of the atomic weights
of all atoms in the compound as represented by its
correct formula
– expressed in amu
• What is the formula weight of H2O?
– 16.00 amu + 2(1.008 amu) = 18.02 amu
• Molar mass – mass of a mole of compound in
grams / mole
– Numerically equal to the formula weight in amu
• What is the molar mass of H2O?
– 18.02 g/mol H2O
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Formula Unit
• Formula unit – smallest
collection of atoms from which
the formula of a compound can
be established
• When calculating the formula
weight (or molar mass) of an
ionic compound, the smallest
unit of the crystal is used
What is the molar mass of (NH4)3PO4?
3(N amu) + 12(H amu) + P amu + 4(O amu)=
3(14.01) + 12(1.008) + 30.97 + 4(16.00)=
149.10 g/mol (NH4)3PO4
Molar Mass
• Molar mass - The mass in grams of 1 mole of
atoms
• What is the molar mass of carbon?
12.01 g/mol C
• This means counting out a mole of carbon
atoms (i.e., 6.022 x 1023) they would have a mass
of 12.01 g
• One mole of any element contains the same
number of atoms, 6.022 x 1023, Avogadro’s number
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The Chemical Equation and the
Information It Conveys
A Recipe For Chemical Change
• Chemical equation - shorthand notation of a
chemical reaction
– Describes all of the substances that react and all
the products that form, physical states, and
experimental conditions
– Reactants – (starting materials) – the substances
that undergo change in the reaction
– Products – substances produced by the reaction
Features of a Chemical Equation
1. Identity of products and reactants must be
specified using chemical symbols
2. Reactants are written to the left of the reaction
arrow and products are written to the right
3. Physical states of reactants and products may
be shown in parentheses
4. Symbol  over the reaction arrow means that
energy is necessary for the reaction to occur
5. Equation must be balanced
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Features of a Chemical Equation
2HgO( s )


2Hg( l ) 
O 2 (g )
Products and reactants must be
specified using chemical symbols
Reactants – written on the left of arrow
Products – written on the right
 – energy is needed
Physical states are shown in parentheses
“Everything” is balanced
•number of atoms
•moles of stuff
•mass
•energy
•charge
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The Experimental Basis of a
Chemical Equation
We know that a chemical equation represents a
chemical change
• One or more substances are changed into
new substances
• Different chemical and physical properties
Evidence of a Reaction Occurring
The following can be visual evidence of a reaction:
•Release of a gas
– CO2 is released when strong acid is placed in a
solution containing CO32- ions
•Formation of a solid (precipitate)
– A solution containing Ag+ ions mixed with a solution
containing Cl- ions
•Heat is produced or absorbed
– Acid and base are mixed together
•Color changes
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Subtle Indications of a Reaction
• Heat or light is absorbed or emitted
• Changes in the way the substances behave in an
electrical or magnetic field before and after a
reaction
• Changes in electrical properties
Writing Chemical Reactions
• We will learn to identify the following patterns
of chemical reactions:
–
–
–
–
combination
decomposition
single-replacement
double-replacement
• Recognizing the pattern will help you write
and understand reactions
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