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Isotopes
• Isotopes are atoms of the same element with
different masses.
• Isotopes have different numbers of neutrons.
© 2012 Pearson Education, Inc.
© 2012 Pearson Education, Inc.
© 2012 Pearson Education, Inc.
Do You Understand Isotopes?
How many protons, neutrons, and electrons are
14
in 6 C ?
6 protons, 8 (14 - 6) neutrons, 6 electrons
How many protons, neutrons, and electrons are
11
in 6 C ?
6 protons, 5 (11 - 6) neutrons, 6 electrons
© 2012 Pearson Education, Inc.
2.3
Exercise
1. Magnesium has three isotopes with mass
numbers 24, 25, and 26. (a) Write the
complete chemical symbol (superscript
and subscript) for each. (b) How many
neutrons are in an atom of each isotope?
2. Give the complete chemical symbol for
the atom that contains 82 protons, 82
electrons, and 126 neutrons.
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Atomic Mass
Atomic and
molecular masses
can be measured
with great accuracy
using a mass
spectrometer.
© 2012 Pearson Education, Inc.
Average Mass
• Because in the real world we use large amounts
of atoms and molecules, we use average
masses in calculations.
• Average mass is calculated from the isotopes of
an element weighted by their relative
abundances.
© 2012 Pearson Education, Inc.
© 2012 Pearson Education, Inc.
Exercise
Naturally occurring chlorine is 75.78% 35Cl
(atomic mass 34.969 amu) and 24.22% 37Cl
(atomic mass 36.966 amu).
Calculate the atomic weight of chlorine.
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© 2012 Pearson Education, Inc.
© 2012 Pearson Education, Inc.
© 2012 Pearson Education, Inc.
© 2012 Pearson Education, Inc.
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Lecture Presentation
Lesson 4
Stoichiometry:
Calculations with Chemical Formulas and
Equations
© 2012 Pearson Education, Inc.
Chemical Equations
Chemical equations are concise
representations of chemical reactions.
Stoichiometry
© 2015 Pearson Education, Inc.
What Is in a Chemical Equation?
CH4(g) + 2O2(g)
CO2(g) + 2H2O(g)
Reactants appear on the left
side of the equation.
Stoichiometry
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What Is in a Chemical Equation?
CH4(g) + 2O2(g)
CO2(g) + 2H2O(g)
Products appear on the right
side of the equation.
Stoichiometry
© 2015 Pearson Education, Inc.
What Is in a Chemical Equation?
CH4(g) + 2O2(g)
CO2(g) + 2H2O(g)
The states of the reactants and products are written
in parentheses to the right of each compound.
(g) = gas; (l) = liquid; (s) = solid;
(aq) = in aqueous solution
© 2015 Pearson Education, Inc.
Stoichiometry
What Is in a Chemical Equation?
CH4(g) + 2O2(g)
CO2(g) + 2H2O(g)
Coefficients are inserted to balance the equation
to follow the law of conservation of mass.
Stoichiometry
© 2015 Pearson Education, Inc.
Why Do We Add Coefficients Instead of
Changing Subscripts to Balance?
• Hydrogen and oxygen can make water
OR hydrogen peroxide:
➢ 2 H2(g) + O2(g) → 2 H2O(l)
➢ H2(g) + O2(g) → H2O2(l)
Stoichiometry
© 2015 Pearson Education, Inc.
Balancing Chemical Equation
Exercise 1:
Balance following equations:
Na(s) + H2O(l) → NaOH(aq) + H2(g)
Stoichiometry
© 2012 Pearson Education, Inc.
Balancing Chemical Equations
Exercise 2
Balance these equations by providing the missing coefficients:
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
Balancing Chemical Equations
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
Balancing Chemical Equations _ANSWERS
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
Six Types of Reactions
•
•
•
•
•
•
Combination reactions
Decomposition reactions
Combustion reactions
Single Displacement
Double Displacement
Neutralization Reaction
Stoichiometry
© 2015 Pearson Education, Inc.
Chemical Equation Symbols
• Here are several symbols used in chemical
equations:
24
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Stoichiometry
Chapter 8
Diatomic Molecules
• Seven nonmetals occur naturally as diatomic
molecules.
• They are hydrogen (H2); nitrogen (N2);
oxygen (O2); and the halogens F2, Cl2, Br2,
and I2.
• These elements are written
as diatomic molecules
when they appear in
chemical
reactions.
25
© 2015 Pearson Education, Inc.
Stoichiometry
Chapter 8
Combination Reactions
• In combination
reactions two or
more substances
react to form one
product.
• Examples:
– 2Mg(s) + O2(g) ⎯⎯→ 2MgO(s)
– N2(g) + 3H2(g) ⎯⎯→ 2NH3(g)
– C3H6(g) + Br2(l) ⎯⎯→ C3H6Br2(l)
© 2012 Pearson Education, Inc.
Stoichiometry
Decomposition Reactions
• In a decomposition
reaction one
substance breaks
down into two or
more substances.
• Examples:
– CaCO3(s) ⎯⎯→ CaO(s) + CO2(g)
– 2KClO3(s) ⎯⎯→ 2KCl(s) + O2(g)
– 2NaN3(s) ⎯⎯→ 2Na(s) + 3N2(g)
Sodium azide is used in air-bags of cars.
© 2012 Pearson Education, Inc.
Stoichiometry
Exercise 3
Write a balanced equation for (a) the combination reaction between lithium metal
and fluorine gas and (b) the decomposition reaction that occurs when solid
barium carbonate is heated (two products form, a solid and a gas).
Solution
Exercise 4
Write a balanced equation for (a) solid mercury(II) sulfide
decomposing into its component elements when heated and (b)
aluminum metal combining with oxygen in the air.
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
Combustion Reactions
• Combustion reactions
are generally rapid
reactions that produce
a flame.
• Combustion reactions
most often involve
hydrocarbons reacting
with oxygen in the air.
• Examples:
– CH4(g) + 2O2(g) ⎯⎯→ CO2(g) + 2H2O(g)
– C3H8(g) + 5O2(g) ⎯⎯→ 3CO2(g) + 4H2O(g)
© 2012 Pearson Education, Inc.
Stoichiometry
Exercise 5: Writing Balanced Equations for Combustion Reactions
Write the balanced equation for the reaction that occurs when
methanol,CH3OH(l), is burned in air.
Exercise 6
Write the balanced equation for the reaction that occurs when
ethanol, C2H5OH(l), burns in air.
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
Single-Replacement Reactions
• A single-replacement reaction is a a reaction
where a more active metal displaces another, less
active metal in a compound.
• If a metal precedes another in
the activity series, it will
undergo a single-replacement
reaction:
Fe(s) + CuSO4(aq) →
FeSO4(aq) + Cu(s)
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
Double-Replacement Reactions
• In a double displacement reaction, two ionic
compounds in aqueous solution switch anions and
produce two new compounds
AX + BZ → AZ + BX
• If either AZ or BX is an insoluble compound, a
precipitate will appear and there is a chemical
reaction.
• If no precipitate is formed, there is no reaction.
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
Double-Replacement Reactions
• Aqueous barium chloride reacts with aqueous
potassium chromate:
2 BaCl2(aq) + K2CrO4(aq) → BaCrO4(s) + 2 KCl(aq)
• From the solubility rules, BaCrO4 is insoluble, so
there is a double-displacement reaction.
• Aqueous sodium chloride reacts with aqueous
lithium nitrate:
NaCl(aq) + LiNO3(aq) → NaNO3(aq) + LiCl(aq)
• Both NaNO3 and LiCl are soluble, so there is no
reaction.
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
Neutralization Reactions
• A neutralization reaction is the reaction of an
acid and a base.
HX + BOH → BX + HOH
• A neutralization reaction produces a salt and
water.
H2SO4(aq) + 2 KOH(aq) → K2SO4(aq) + 2 H2O(l)
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
Critical Thinking: Household Chemicals
• Many common household items contain familiar
chemicals
– vinegar is a solution of acetic acid
– drain and oven cleaners contain sodium hydroxide
– car batteries contain sulfuric acid
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
Chapter 8
© 2012 Pearson Education, Inc.
35
Chapter Summary
• There are 4 ways to tell if a chemical reaction
has occurred:
1. A gas is detected.
2. A precipitate is formed.
3. A permanent color change is seen.
4. Heat or light is given off.
• An exothermic reaction gives off heat and an
endothermic reaction absorbs heat.
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
Chapter Summary, continued
• There are 7 elements that exist as diatomic
molecules:
– H2, N2, O2, F2, Cl2, Br2, and I2
• When we balance a chemical equation, the
number of each type of atom must be the same on
both the product and reactant sides of the
equation.
• We use coefficients in front of compounds to
balance chemical reactions.
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
Chapter Summary, continued
• There are 5 basic types of chemical reactions.
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
Chapter Summary, continued
• In combination reactions, two or more smaller
molecules are combined into a more complex
molecule.
• In a decomposition reaction, a molecule breaks
apart into two or more simpler molecules.
• In a single-replacement reaction, a more active
metal displaces a less active metal according to
the activity series.
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
Chapter Summary, continued
• In a double-replacement reaction, two aqueous
solutions produce a precipitate of an insoluble
compound.
• The insoluble compound can be predicted based
on the solubility rules.
• In a neutralization reaction, ann acid and a base
react to produce a salt and water.
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
TRY THIS!
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
TRY THIS!
Answers to Exercises
1. single displacement
2. double displacement
3. decomposition
4. combination
5. single displacement
6. combustion
7. double displacement
8. combination
9. decomposition
10. double displacement
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
TRY THIS!
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
Formula
Weights
Stoichiometry
© 2012 Pearson Education, Inc.
Micro World
atoms & molecules
Macro World
grams
Atomic mass is the mass of an atom in
atomic mass units (amu)
By definition:
1 atom 12C “weighs” 12 amu
On this scale
1H
= 1.008 amu
16O
= 16.00 amu
3.1
Formula Weight (FW)
• A formula weight is the sum of the atomic
weights for the atoms in a chemical formula.
• So, the formula weight of calcium chloride,
CaCl2, would be
Ca: 1(40.08 amu)
+ Cl: 2(35.453 amu)
110.99 amu
• Formula weights are generally reported for
ionic compounds.
Stoichiometry
© 2012 Pearson Education, Inc.
Molecular Weight (MW)
• A molecular weight is the sum of the atomic
weights of the atoms in a molecule.
• For the molecule ethane, C2H6, the molecular
weight would be
C: 2(12.011 amu)
+ H: 6(1.00794 amu)
30.070 amu
Stoichiometry
© 2012 Pearson Education, Inc.
Exercise 7: Calculating Formula Weights
Calculate the formula weight of (a) sucrose, C12H22O11
(table sugar), and (b) calcium nitrate, Ca(NO3)2.
Exercise 8
Calculate the formula weight of (a) Al(OH)3 and (b)
CH3OH.
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
Percent Composition
One can find the percentage of the
mass of a compound that comes from
each of the elements in the compound
by using this equation:
(number of atoms)(atomic mass)
% Element =
(FW of the compound)
x 100
Stoichiometry
© 2012 Pearson Education, Inc.
Percent Composition
So the percentage of carbon in ethane
C2H6 is
(2)(12.011 amu)
%C =
(30.070 amu)
24.022 amu
x 100
=
30.070 amu
= 79.887%
Stoichiometry
© 2012 Pearson Education, Inc.
Exercise 9: Calculating Percentage Composition
Calculate the percentage of carbon, hydrogen, and oxygen
(by mass) in C12H22O11.
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
Avogadro’s
Number and
Moles
Stoichiometry
© 2012 Pearson Education, Inc.
Avogadro’s Number
• In a lab, we cannot
work with individual
molecules. They are
too small.
• 6.02 × 1023 atoms
or molecules is an
amount that brings
us to lab size. It is
ONE MOLE.
• One mole of 12C has
a mass of 12.000 g.
© 2015 Pearson Education, Inc.
Stoichiometry
Avogadro’s Number
6.02 x 1023 particles
1 mole
or
1 mole
6.02 x 1023 particles
Molar Mass
• By definition, a molar mass is the mass
of 1 mol of a substance (i.e., g/mol).
– The molar mass of an element is the mass
number for the element that we find on the
periodic table.
– The formula weight (in amu’s) will be the
same number as the molar mass (in
g/mol).
Stoichiometry
© 2012 Pearson Education, Inc.
A Moles of Particles
Contains 6.02 x 1023 particles
1 mole C
= 6.02 x 1023 C atoms
1 mole H2O
= 6.02 x 1023 H2O molecules
1 mole NaCl
= 6.02 x 1023 Na+ ions and
6.02 x 1023 Cl– ions
Examples of Moles
Moles of elements
1 mole Mg = 6.02 x 1023 Mg atoms
1 mole Au = 6.02 x 1023 Au atoms
Moles of compounds
1 mole NH3 = 6.02 x 1023 NH3 molecules
1 mole C9H8O4
= 6.02 x 1023 aspirin molecules
Exercise 10: How many atoms are in 0.551 g of
potassium (K) ?
1 mol K = 39.10 g K
1 mol K = 6.022 x 1023 atoms K
1 mol K
6.022 x 1023 atoms K
0.551 g K x
x
=
1 mol K
39.10 g K
8.49 x 1021 atoms K
Using Moles
Moles provide a bridge from the molecular
scale to the real-world scale.
Stoichiometry
© 2012 Pearson Education, Inc.
Practice: Moles to
Particles/Molecules
Conversion
MOLES
PARTICLES/ MOLECULES
How many molecules are in 9.3 moles of SiH4?
Determine how many particles are in 2.15 moles of gold.
Determine how many particles are in 0.151 moles of nitrogen
oxide.
Practice:
Particles/Molecules
to Moles
Conversion
PARTICLES/MOLECULES
MOLES
How many moles in 3.01 x 1026 molecules of carbon dioxide?
How many moles of ammonia are in 1.20 x 1025 molecules?
Calculate the number of moles of 8.92 x 1023 atoms of barium.
Practice: Moles
to Mass
Conversion
MOLES
MASS
What is the mass of 3.35 moles of calcium?
What is the mass of 0.100 moles of cream of tartar
(KHC4H4O5)?
What is the mass of 0.15 moles of CuSO4?
Practice: Mass
to Mole
Conversion
Exercise 11: Converting Grams to Moles
Calculate the number of moles of glucose (C6H12O6) in
5.380 g of C6H12O6.
Exercise 12
How many moles of sodium bicarbonate (NaHCO3)
are in 508 g of NaHCO3?
Chemistry, The Central Science, 12th Edition
Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
© 2012 Pearson Education, Inc.
MASS
MOLE
How many moles are in 19.9 g of ammonia?
How many moles are in 76.0 g of CaBr2?
How many moles of titanium are contained in 71.4 g?
Mole Relationships
• One mole of atoms, ions, or molecules contains
Avogadro’s number of those particles.
• One mole of molecules or formula units contains
Avogadro’s number times the number of atoms or
ions of each element in the compound.
Stoichiometry
© 2012 Pearson Education, Inc.
Practice: Mass to
Particle/Molecules
Conversion
Stoichiometry
MASS
PARTICLES/MOLECULES
Calculate the number of atoms in 1.24 g of lithium.
Calculate the number of atoms in 346 g of zinc.
Calculate the number of atoms in 115 g of nickel.
Practice:
Particle/Molecules
To Mass
Conversion
PARTICLES/MOLECULES
MASS
What is the mass of 1.33 x 1024 atoms of argon?
Determine the mass of 5.65 x 1024 atoms of Se.
How many grams in 1.00 x 1023 molecules of Mg(C2H3O2)2?
Recall: Chemical Formula
Stoichiometry
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Finding
Empirical
Formulas
Stoichiometry
© 2012 Pearson Education, Inc.
Empirical
The name empiric derives from Latin empīricus, itself from
Greek empeirikós, meaning "based on observation (of medical
treatment), experienced." The root of the Greek word (-peiros) is
a derivative of peîra, meaning "attempt, trial, test."
Stoichiometry
© 2012 Pearson Education, Inc.
Formula
from Latin, diminutive
of forma ‘shape, mold’.
Stoichiometry
© 2012 Pearson Education, Inc.
Calculating Empirical Formulas
One can calculate the empirical formula from
the percent composition.
Stoichiometry
© 2012 Pearson Education, Inc.
Percent Composition
One can find the percentage of the
mass of a compound that comes from
each of the elements in the compound
by using this equation:
(number of atoms)(atomic mass)
% Element =
(FW of the compound)
x 100
Stoichiometry
© 2012 Pearson Education, Inc.
Recall: Percent Composition
One can calculate the empirical formula from
the percent composition.
Let’s recall : What percent of FeSO4 • 6H2O is Fe?
Stoichiometry
© 2012 Pearson Education, Inc.
Calculating Empirical Formulas
One can calculate the empirical formula from
the percent composition.
Stoichiometry
© 2012 Pearson Education, Inc.
Calculating Empirical Formulas
Stoichiometry
© 2012 Pearson Education, Inc.
Calculating Empirical Formulas
Example 1: The compound para-aminobenzoic acid
(you may have seen it listed as PABA on your bottle of
sunscreen) is composed of carbon (61.31%), hydrogen
(5.14%), nitrogen (10.21%), and oxygen (23.33%).
Find the empirical formula of PABA.
Stoichiometry
© 2012 Pearson Education, Inc.
Calculating Empirical Formulas
Assuming 100.00 g of para-aminobenzoic acid,
C:
H:
N:
O:
1 mol
12.01 g
1 mol
5.14 g x
1.01 g
1 mol
10.21 g x
14.01 g
1 mol
23.33 g x
16.00 g
61.31 g x
= 5.105 mol C
= 5.09 mol H
= 0.7288 mol N
= 1.456 mol O
Stoichiometry
© 2012 Pearson Education, Inc.
Calculating Empirical Formulas
Calculate the mole ratio by dividing by the smallest number
of moles:
C:
5.105 mol
0.7288 mol
= 7.005  7
H:
5.09 mol
0.7288 mol
= 6.984  7
N:
0.7288 mol
0.7288 mol
= 1.000
O:
1.458 mol
0.7288 mol
= 2.001  2
© 2012 Pearson Education, Inc.
Stoichiometry
Calculating Empirical Formulas
These are the subscripts for the empirical formula:
C7H7NO2
Stoichiometry
© 2012 Pearson Education, Inc.
Calculating Empirical Formulas
1. A compound is found to contain 26.56% potassium,
35.41% chromium, and the remainder oxygen. Find
its empirical formula.
2. A compound is found to contain 63.52 % iron and
36.48% sulfur. Find its empirical formula.
3. A 60.00 g sample of tetraethyllead, a gasoline
additive, is found to contain 38.43 g lead, 17.83 g
carbon, and 3.74 g hydrogen. Find its empirical formula.
Stoichiometry
© 2012 Pearson Education, Inc.
Seatwork
1. A 170.00 g sample of an unidentified
compound contains 29.84 g sodium, 67.49 g
chromium, and 72.67 g oxygen. Find the
empirical formula.
2. The percent composition of an unknown
substance is 75.42 % Carbon, 6.63 %
Hydrogen, 8.38 % Nitrogen, and 9.57 %
Oxygen. Find its empirical formula.
Stoichiometry
© 2012 Pearson Education, Inc.
Evaluation
Find its empirical formula!
1. 26.4 % Carbon 3.3 % Hydrogen 70.3 % Oxygen.
2. 81.8 grams Carbon ;18.2 grams Hydrogen.
3. 20.2 % Sodium 37.6 % Sulfur 42.2 % Oxygen
4. 8.81 g Carbon 91.2 g Chlorine
5. Determine the empirical `of a compound
composed of 18.24 g Carbon, 0.51 g Hydrogen,
Stoichiometry
and 16.91 g Fluorine
© 2012 Pearson Education, Inc.
Molecular Formulas
• The molecular formula is a multiple of the
empirical formula
• To determine the molecular formula you
need to know the empirical formula and
the molar mass of the compound
Stoichiometry
Example #3
Determine the Molecular Formula of Benzopyrene
if it has a molar mass of 252 g and an
empirical formula of C5H3
Determine the empirical formula
• May need to calculate it as previous
C5H 3
Determine the molar mass of the
empirical formula
5 C = 60.05 g, 3 H = 3.024 g
C5H3 = 63.07 g
Stoichiometry
Example #3
Determine the Molecular Formula of Benzopyrene
if it has a molar mass of 252 g and an
empirical formula of C5H3
Divide the given molar mass of the
compound by the molar mass of the
empirical formula
– Round to the nearest whole number
252 g
=4
63.07 g
Stoichiometry
Seatwork: Finding Molecular Formula
•1. Determine the molecular formula of the
compound with an empirical formula of CH and a
formula mass of 78.110 u.
•2. Determine the molecular formula of a
compound with an empirical formula of NH2 and a
formula mass of 32.06 u.
Stoichiometry
© 2012 Pearson Education, Inc.
Evaluation
Finding MOLECULAR FORMULA!
1. The percent composition of an unknown
substance is 75.42 % Carbon, 6.63 % Hydrogen,
8.38 % Nitrogen, and 9.57 % Oxygen. If its molar
mass is 334.0 g/mol what is its empirical and
molecular formula?
2. A compound with a molar mass of 544.0 g/mol is
made up of 26.5 grams Carbon, 2.94 grams
Hydrogen, and 70.6 grams Oxygen. What is its
empirical and molecular formula?
Stoichiometry
© 2012 Pearson Education, Inc.
1. Calculate the mass percent of carbon, nitrogen and oxygen
in acetamide, C2H5NO.
2. A 50.51 g sample of a compound made from phosphorus
and chlorine is decomposed. Analysis of the products
showed that 11.39 g of phosphorus atoms were produced.
What is the empirical formula of the compound?
3. What’s the empirical formula of a molecule containing
18.7% lithium, 16.3% carbon, and 65.0% oxygen? If the
molar mass of the compound is 73.8 grams/mole, what’s
the molecular formula?
4. A component of a protein called serine has an approximate
molar mass of 100 g/mole. If the percent composition is as
follows, what is the empirical and molecular formula of
serine?
C = 34.95 % H= 6.844 % O = 46.56 % N= 13.59 %
5. The compound benzamide has the following percent
composition. What is the empirical formula?
C = 69.40 % H= 5.825 % O = 13.21 % N= 11.57 %
Stoichiometry
© 2012 Pearson Education, Inc.