Chapter 7
Section 3 Using Chemical
Formulas
Lesson Starter
 The chemical formula for water is H2 O.
 How many atoms of hydrogen and oxygen are there
in one water molecule?
 How might you calculate the mass of a water
molecule, given the atomic masses of hydrogen and
oxygen?
 In this section, you will learn how to carry out these
and other calculations for any compound.
Chapter 7
Section 3 Using Chemical
Formulas
Objectives
 Calculate the formula mass or molar mass of any
given compound.
 Use molar mass to convert between mass in grams
and amount in moles of a chemical compound.
 Calculate the number of molecules, formula units, or
ions in a given molar amount of a chemical
compound.
 Calculate the percentage composition of a given
chemical compound.
Chapter 7
Section 3 Using Chemical
Formulas
 Chemical formulas can be used to determine:
 formula mass
 molar mass
 percentage composition
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Chapter 7
Section 3 Using Chemical Formulas
Formula Masses
 The formula mass is the sum of the average
atomic masses of all in its formula. (units are
amu)
 example:
formula mass of water, H2O
average atomic mass of H: 1.01 amu
average atomic mass of O: 16.00 amu
average mass of H2O molecule: 18.02 amu
Section 3 Using Chemical
Formulas
Chapter 7
Formula Masses, continued
Sample Problem 7-6 p. 222
Find the formula mass of potassium chlorate, KClO3 .
Practice Problems p. 222
1. Find the formula mass of each of the following:
a. H2SO4
b. Ca(NO3)2
c. (PO4)3Image:
http://www.chm.bris.ac.uk/motm/h2so4/bart.
gif
d. MgCl2
Chapter 7
Section 3 Using Chemical
Formulas
Molar Masses
 The molar mass of a substance is equal to the mass
in grams of one mole, or approximately 6.022 x1023
particles, of the substance.
 A compound’s molar mass is numerically equal to its
formula mass, but the units are in grams per mol.
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Chapter 7
Section 3 Using Chemical
Formulas
Molar Masses, continued
 One mole of water molecules contains exactly two
moles of H atoms and one mole of O atoms. The
molar mass of water is calculated as follows.
molar mass of H2O molecule: 18.02 g/mol
 A compound’s molar mass is numerically equal to its
formula mass.
Chapter 7
Section 3 Using Chemical
Formulas
Molar Masses, continued
Sample Problem 7-7 p. 223
What is the molar mass of barium
nitrate, Ba(NO3)2 ?
Practice Problems p. 223
1.
2. Find the molar
mass of each of the
compounds listed in
item 1.
How may moles of atoms of each
element are there in one mole of
the following compounds?
a.
Al2 S 3
b.
NaNO3
c.
Ba(OH)2
Chapter 7
Section 3 Using Chemical
Formulas
Molar Mass as a Conversion Factor
Amount in moles x molar mass (g/mol)
= mass in grams
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Section 3 Using Chemical
Formulas
Chapter 7
Molar Mass as a Conversion Factor, continued
Sample Problem 7-8 p. 224
What is the mass in grams of 2.50 mol of oxygen gas?
Sample Problem 7-9 p. 225
Ibuprofen, C13H 18O 2, is the active ingredient in many
nonprescription pain relievers. Its molar mass is
206.31 g/mol.
a. If the tablets in a bottle contain a total of 33 g of ibuprofen, how
many moles of ibuprofen are in the bottle?
b. How many molecules of ibuprofen are in the bottle?
c. What is the total mass in grams of carbon in 33 g
of ibuprofen?
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Section 3 Using Chemical
Formulas
Chapter 7
Percentage Composition
 The percentage by mass of each element in a
compound is known as the percentage composition
of the compound.
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Chapter 7
Section 3 Using Chemical
Formulas
Percentage Composition of Iron Oxides
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Chapter 7
Section 3 Using Chemical
Formulas
Percentage Composition, continued
Sample Problem 7-10 p. 227
Find the percentage composition of copper(I) sulfide,
Cu2S.
Image:www.galleries.com/minerals/sulfides/chalcoci/
chalcoci.jpg
Chapter 7
Section 3 Using Chemical
Formulas
Percentage Composition, continued
Practice Problem #1 p. 228
1.
Find the percentage compositions of the following:
a.
PbCl2
b.
Ba(NO3 ) 2
Image:www.galleries.com/minerals/sulfides/chalcoci/
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