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Julien
Name _________________________
Period _____ Date _______________
Chemical Quantities
Chapter 7
Conservation of Matter and Stoichiometry
3. The conservation of atoms in chemical reactions leads to the principle of conservation of matter
and the ability to calculate the mass of products and reactants. As a basis for understanding this
concept:
b. Students know the quantity one mole is set by defining one mole of carbon 12 atoms to have a mass
of exactly 12 grams.
c. Students know one mole equals 6.02 x 1023 particles (atoms or molecules).
d. Students know how to determine the molar mass of a molecule from its chemical formula and a
table of atomic masses and how to convert the mass of a molecular substance to moles, number of
particles, or volume of gas at standard temperature and pressure.
e. Students know how to calculate the masses of reactants and products in a chemical reaction from the
mass of one of the reactants or products and the relevant atomic masses.
f.* Students know how to calculate percent yield in a chemical reaction.
I. The Mole.
Read Section 7.1 in your eText! I will use Avogadro’s number to determine the number of
particles in a given number of moles.
Do the Using Avogadro’s Number Tutorial!
A. Avogadro’s number.
1. Mole (mol)—
2. A mole is a unit, like a dozen, a ream or a gross, used to group items in convenient amounts.
3. We call it Avogadro’s number because it is named in honor of an Italian physicist, Amedeo
Avogadro and it can refer to ions, molecules, formula units, atoms, etc.
a. Formula units—
b. Molecules —
3. It becomes a conversion factor because it is an equality.
1 mol
6.022 ´ 10 23
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or
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6.022 ´ 10 23
1 mol
Chapter 7 Notes from Basic Chemistry
Calculating the Atoms or Molecules of a Substance
STEP 1:
STEP 2:
STEP 3:
STEP 4:
Determine the given number of moles.
Write a plan to convert moles to atoms or molecules.
Use Avogadro’s number to write conversion factors.
Set up problem to convert given moles to atoms or molecules.
Problem 1: How many moles of water, H2O, contain 2.60 x 1023 molecules of water?
Do the Moles and the Chemical Formula Tutorial and the Conversions Involving Moles
Tutorial!
B. Moles of elements in a formula.
1. A chemical formula tells how many atoms of each type are in a compound and it tells how
many moles of atoms of each type are in a mole of that compound.
2. Sulfuric acid, H2SO4, has two atoms (two moles) of hydrogen, one atom (one mole) of sulfur,
and four atoms (four moles) of oxygen in one molecule (one mole) of sulfuric acid.
3. This makes a set of equivalencies for mathematical conversions.
2 mol H
1 mol H 2SO4
or
1 mol H 2SO 4
2 mol H
1 mol S
1 mol H 2SO4
or
1 mol H 2SO 4
1 mol S
4 mol O
1 mol H 2SO4
or
1 mol H 2SO 4
4 mol O
Problem 2: How many moles of aspirin, C9H8O4, contain 0.480 mol of O?
Problem 3: How many SO42– ions are in 2.50 mol of Fe2(SO4)3, a compound used in water and sewage
treatments?
You are now able to do problems 7.2 and 7.4 in your first homework assignment.
II. Molar Mass.
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Chapter 7 Notes from Basic Chemistry
Read Section 7.2 in your eText! Given the chemical formula of a substance, I will calculate
its molar mass.
A. Molar mass of a compound.
1. Molar mass—
2. The molar mass is equal to the atomic mass in grams.
3. An atom of carbon-12 has a mass of 12.01 amu and a mole of carbon-12 atoms has a mass of
12.01 grams.
Problem 4: Calculate the molar mass of salicylic acid, C7H6O3, which is used to make aspirin.
You are now able to do problems 7.10 and 7.12 in your first homework assignment and problems 7.58 and
7.60 in your second homework assignment.
III. Calculations using Molar Mass.
Read Section 7.3 in your eText! Given the formula of moles of a substance, I will calculate
the mass in grams; given the mass, I will calculate the number of moles.
Calculating the Moles (or Grams) of a Substance from the Grams (or Moles)
STEP 1:
STEP 2:
STEP 3:
STEP 4:
Determine the given number of moles (or grams).
Write a plan to convert moles to grams (or grams to moles).
Determine the molar mass and write conversion factors.
Set up problem to convert given moles to grams (or grams to moles).
A. Molar mass calculations.
1. If one mole of carbon is equal to 12.01 g, then we can establish a conversion factor.
1 mol C
12.01 g C
or
12.01 g C
1 mol C
2. Conversion factors can also be written for molecules or formulas.
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Chapter 7 Notes from Basic Chemistry
1 mol H 2O
18.016 g H 2O
or
18.016 g H 2O
1 mol H 2 O
3. We can now convert from moles to grams and from grams to moles.
Do the Converting Between Grams and Moles Tutorial!
Problem 5: Calculate the number of grams of gold (Au) present in 0.124 mol of gold.
Problem 6: One gel cap of an antacid contains 311 mg of CaCO3 and 232 mg of MgCO3. In a recommended
dosage of two gel caps, how many moles each of CaCO3 and MgCO3 are present?
Converting Grams to Particles
STEP 1:
STEP 2:
STEP 3:
STEP 4:
Determine the given number of grams.
Write a plan to convert grams to particles.
Write the conversion factors for molar mass and Avogadro’s number.
Set up problem to convert grams to molecules to particles.
Problem 7: In the body, caffeine, which has a formula of C8H10N4O2, acts as a stimulant and diuretic. How
many moles of nitrogen are in 2.50 g of caffeine?
You are now able to do problems 7.14, 7.18, and 7.22 in your first homework assignment and 7.56 in your
second homework assignment.
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Chapter 7 Notes from Basic Chemistry
IV. Percent Composition and Empirical Formula.
Read Section 7.4 in your eText! Given the formula of a compound, I will calculate the
percent composition; from the percent composition, I will determine the empirical formula of
a compound.
Do the Mass Percent Tutorial!
A. Empirical formulas.
1. Percent composition—
2. The percentage of each element is determined by the following formula:
Mass percent of each element in a compund =
mass of each element
´ 100%
molar mass of compound
Calculating Percent Composition
STEP 1: Determine the total mass of each element in the molar mass of a formula.
STEP 2: Divide the total mass of each element by the molar mass and multiply by 100%.
Problem 8: The organic compound ethylene glycol, C2H6O2, used as automobile antifreeze, is a sweet-tasting
liquid, which is toxic to humans and animals. What is the percent composition by mass of
ethylene glycol?
3.
4.
5.
5.
Molecular formula—
Empirical formula—
Benzene, C6H6, has an empirical formula of CH.
The empirical formula of a compound is determined by converting the number of grams of each
element to moles and finding the lowest whole-number ratio to use as subscripts.
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Calculating Empirical
Page 5 Formula Chapter 7 Notes from Basic Chemistry
STEP 1: Calculate the moles of each element.
STEP 2: Divide by the smallest number of moles.
STEP 3: Use the lowest whole-number ratio of moles as subscripts.
Problem 9: Phosphine is a highly toxic compound used for pest and rodent control. If a sample of phosphine
contains 0.456 g of P and 0.0440 g of H, what is its empirical formula?
Problem 10: Sulfate of potash is the common name of a compound used in fertilizers to supply potassium and
sulfur. What is the empirical formula of this compound if it has a percent composition of 44.9%
K, 18.4% S, and 36.7% O?
B. Converting decimal numbers to whole numbers.
1. Sometimes the result of dividing by the smallest number o moles gives a decimal instead of a
whole number.
2. Round decimal values that are very close to whole numbers can be rounded off to the whole
number.
3. A decimal that is greater than 0.1 or less than 0.9 should not be rounded off.
4. Multiply the numbers that shouldn’t be rounded off by using the table below.
Decimal
0.20
0.25
0.33
0.50
0.67
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Multiply by
5
4
3
2
3
Example
1.20 x 5
2.25 x 4
1.33 x 3
2.50 x 2
1.67 x 3
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=
=
=
=
=
Whole Number
6
9
4
5
5
Chapter 7 Notes from Basic Chemistry
Problem 11: An organic compound called glyoxylic acid is used by plants and bacteria to convert fats into
glucose. What is the empirical formula of glyoxylic acid if it contains 32.5% C, 2.70% H, and
64.8% O?
You are now able to do problems 7.26, 7.28, and 7.32 in your first homework assignment and problems 7.36
and 7.38 in your second homework assignment.
V. Molecular Formulas.
Read Section 7.5 in your eText! I will determine the molecular formula of a substance from
the empirical formula and molar mass.
Do the Empirical and Molecular Formulas Tutorial and the Formula Mass Tutorial!
A. Relating empirical and molecular formulas.
1. The molecular formula is the product of a small integer times the empirical formula.
2. The molecular mass is the product of a small integer times the empirical mass.
3. Dividing the molecular mass by the empirical mass will give you the value of the small integer.
4. Multiplying the empirical formula by the small integer will give you the molecular formula.
B. Calculating a molecular formula.
1. Calculate the empirical mass from the empirical formula by adding the masses of all the
elements in the empirical formula.
2. Divide the molecular mass, which will be given in the problem, by the empirical mass to get the
small integer.
3. Multiply the empirical formula by the small integer to obtain the molecular formula.
Calculating Percent Composition
STEP 1: Calculate the empirical formula mass.
STEP 2: Divide the molar mass by the empirical formula mass to obtain a small integer.
STEP 3: Multiply the empirical formula by the small integer to obtain the molecular formula.
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Chapter 7 Notes from Basic Chemistry
Problem
12: The insecticide lindane has a percent composition of 24.78% C, 2.08% H, and 73.14% Cl. If its molar
mass is about 291 g/mol, what is its molecular formula?
Formula
Mass
Molecular
291 g/mol
Empirical
You are now able to do problems 7.44, 7.47, 7.50, and 7.70 in your second homework assignment.
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Chapter 7 Notes from Basic Chemistry