Download Unit 7 Chemical Composition: he Mole We Need to Count atoms

Introductory Chemistry
Nivaldo Tro
Unit 7
Chemical Composition:
The Mole
(Chapter 6)
2006, Prentice Hall
We Need to Count atoms
Airbags are inflated by a chemical reaction:
2 NaN3(s)
electrical
decomposition
3 N2(g) + 2 Na(s)
Each airbag needs the right
amount of NaN3 (sodium azide)
to fill the bag with nitrogen gas
Grouping Items for Counting
•  The size of the group depends on how many
items we use at a time
1 dozen=12
gross = 12x12
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A Mole is the Counting Unit for Atoms
1 mole = “chemists dozen”
1 pair = 2 items
1 dozen = 12 items
1 gross = 144 items
1 mole =
1 mole eggs = 6.02 x 1023 eggs
1 mole H atoms = 6.02 x 1023 H atoms
1 mole S atoms = 6.02 x 1023 S atoms
Average Atomic Mass
The original atomic mass scale used 1 H atom as the base.
The average atomic mass of 1 H atom = 1.008 amu. The
average atomic masses of all other atoms is relative.
1 H atom = 1 proton = 1.7 x 10-24 g =
1 atomic mass unit
1 C atom = 12 x 1.7 x 10-24 g =
2 x 10-23 g = 12 amu
How much does 1 mole of atoms weigh?
Avogadro’s number was conveniently chosen so that 1 mole of H atoms weighs 1.00 g
1 C atom weighs 12 amu
1 mole of C atoms weighs 12 g
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Atomic masses have the same relative ratio
as molar masses.
1 zinc atom
1 iodine molecule
Moles to Grams --> Grams to Moles
– 
Sample A
Sample B
1 mol H atoms
6.02 x 1023 atoms
Mass = 1.008g
? mol H atoms
? atoms
Mass = 0.500g
How to calculate the number of moles in 0.500g.
1. 
Write down the mass of the sample with units and write down the variable with a
unit that you are trying to convert to.
2. 
Write the conversion factor (molar mass of the element)
3. 
SMILE to get your answer
The Mole Review
Mass
Particles
g/mol
1 mol/
6.02 x 1023
mol/g
6.02 x 1023/
1mol
Moles
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Moles --> Grams Grams --> Moles
•  Aluminum (Al), a metal with a high strength-to-weight ratio
and a high resistance to corrosion, is often used for
structures such as high-quality bicycle frames. Compute the
number of moles of atoms in a 10.0g sample of aluminum.
Moles --> Grams Grams --> Moles
•  Aluminum (Al), a metal with a high strength-to-weight ratio
and a high resistance to corrosion, is often used for
structures such as high-quality bicycle frames. Compute the
total number of aluminum atoms in a 10.0g sample of
aluminum.
Calculations with molar mass
Moles  Grams
•  A Si chip used in an integrated circuit of a
microcomputer has a mass of 5.68mg. How many silicon
atoms (Si) are present in this chip?
Set up the question: 5.68 mg Si = ? g Si
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Mole Review
1. How many moles of Cl are in a 25 g sample of Cl atoms?
2. How many grams of Fe are in 105 moles of Fe?
3. How many moles of Hg are in a 7 ug sample of Hg?
4. How many moles of Cr are in a 1000 kg sample of Cr? How
many atoms of Cr is this?
Molar Mass of a Compound
Molar mass
•  Molar mass – the mass in grams of one mole of a substance. To
calculate the molar mass of a compound, find the number of grams of each
element in one mole of the compound. Then add the masses of the elements
in the compound.
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Molar Mass
One molecule of SO3 has a mass of 80.1 amu. One mol of
SO3 has a mass of 80.1 g.
Molar mass
Molar mass of sulfur dioxide:
SO2
Problem
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Problem
•  Calcium carbonate, CaCO3, is the principle mineral found in
limestone, marble, chalk, pearls, and the shells of marine
animals such as clams.
1. Calculate the molar mass of calcium carbonate.
2. A certain sample of calcium carbonate contains 4.86 mol.
What is the mass in grams of this sample?
Problem
•  Isopentyl acetate, C7H14O2, the compound responsible for
the scent of bananas, can be produced commercially.
Interestingly, bees release about 1 ug of this compound
when they sting. This attracts other bees, which then join
the attack. How many moles and how many molecules of
isopentyl acetate are released in a typical bee sting?
Introductory Chemistry
Nivaldo Tro
Unit 7
Chemical Composition:
Percent Composition
(Chapter 6)
2006, Prentice Hall
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Percent Composition
•  It helps to know the percents of the components in a shirt
because they affect how warm it is, whether it will need to
be ironed, and how it should be cleaned. You will learn how
the percents of the elements in a compound are important in
chemistry.
Percent Composition from Mass Data
•  The relative amounts of the elements in a compound are
expressed as the percent composition or the percent by
mass of each element in the compound.
Percent Composition
•  The percent by mass of an element in a compound is
the number of grams of the element divided by the
mass in grams of the compound, multiplied by
100%.
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Percent Composition from a Formula
Percent Composition by Mass
Practice
1. 
What is the percent by mass of H and the percent by mass
of O in water?
2. 
A compound with a mass of 0.237 g has 0.0948 g C,
0.1264 g O, and 0.0158 g H. What is the % composition
of each element?
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Introductory Chemistry
Nivaldo Tro
Unit 7
Chemical Composition:
Empirical/Molecular Formulas
(Chapter 6)
2006, Prentice Hall
Empirical vs Molecular Formulas
•  Ethyne (C2H2) is a gas used in welder’s torches. Styrene (C8H8) is used in making polystyrene. •  These two compounds of carbon have the same empirical formula (CH) but different molecular formulas.
Empirical Formula
•  The empirical formula gives the.
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Calculating Empirical Formulas
Determining Empirical Formulas
Molecular Formula
–  The molecular formula of a compound is either the
same as its experimentally determined empirical
formula,
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Molecular Formulas
Methanal, ethanoic acid, and glucose all have the same empirical formula—CH2O.
Finding the Molecular Formula
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Practice
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