Topic 5
Chemical Formulas and Equations
Lesson 1: Interpretation of Chemical Formulas
Introduction
Chemical formulas are used to represent the compositions of elements and
compounds (pure substances). A chemical formula expresses the qualitative
and quantitative compositions of a substance.
Qualitative information of a formula shows the types of atoms (or ions) that
make up the substance.
Quantitative information of a formula shows how many of each atom (or
ion) is in the formula. The number of each atom in a formula is shown with a
subscript. A subscript in chemical formulas is the whole number written to
the bottom right of each atom in a formula.
H2SO4
Counting Atoms in Formulas
subscripts
Qualitative and quantitative information of a formula can be determined by
counting how many of each atom is in the formula. Three examples are given
below.
Types of formulas
Simple formula
Formula with
parentheses
Formula of
hydrates
Example formula
# of each atom
H2SO4
(NH4)2O
CuSO4 . 5H2O
Total # of atoms
2 H atoms
1 S atom
4 O atoms
Three different
atoms.
7 total atoms
2 N atoms (2 x 1)
8 H atoms (4 x2)
1 O atom
Three different
atoms.
11 total atoms
1 Cu atom
1 S atom
9 O atoms (4 + 5)
10 H atoms (5 x 2)
Four different
atoms.
21 total atoms
Counting ratio of ions in formulas
Another way of expressing the qualitative and quantitative compositions of a
substance is to determine the ratio of ions in the formula.
Type of formula
Binary ionic
Example formula
CaCl2
calcium chloride
Polyatomic formula
(with parentheses)
Ions in formula
Ca2+
Ratio of ions
1 Ca2+ : 2 Cl-
calcium ion chloride ion
Al2(SO4)3
aluminum sulfate
Cl-
Al3+
SO42-
2 Al3+ : 3 SO42-
aluminum ion sulfate ion
polyatomic formula
KNO3
(without parentheses) potassium nitrate
K+
NO3-
1 K+ : 1 NO3-
potassium ion
nitrate ion
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Topic 5
Chemical Formulas and Equations
Lesson 2: Types of Chemical Formulas
There are three types of chemical formulas that are used to show
compositions of substances.
In this lesson, you will learn about molecular formulas, empirical formulas,
and structural formulas.
Types of Formulas:
A molecular formula is a formula showing the true composition of a
known substance.
A structural formula is a formula showing how atoms of a substance are
bonded together.
An empirical formula is a formula in which the elements are in the
smallest whole-number ratio.
Examples of the three formulas are shown below:
ethane
Molecular
C2H6
H
Structural
Empirical
H2O
H
H–C –C–H
H
water
O
/ \
H
H
H
CH3
H2O
C2H6 is reduced to CH3 by dividing each subscript of the formula by 2
(Greatest Common Factor of 2 and 6)
C6H12 O6 can be reduced to CH2O by dividing each subscript of the
formula by 6 (Greatest Common Factor of 6,12, and 6)
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Topic 5
Chemical Formulas and Equations
Lesson 3 – Chemical Nomenclature
Introduction:
There are millions of known chemical substances, and many more are being
discovered.
Chemical nomenclature refers to the systematic rules for naming and writing
formulas of chemical substances. The International Union of Pure and Applied
Chemistry (IUPAC) is an organization that makes recommendations as to how
chemical substances are named.
In this lesson, you will learn how to apply IUPAC rules to writing formulas and
names for compounds in different classes of inorganic compounds.
Chemical Formulas
A chemical formula is correctly written for a known substance when both the
qualitative and quantitative information of the formula are both correct. This to
say that:
. Element (or ion) symbols in a formula must all be correct for the substance.
. Subscripts of the elements (or ions) in a formula must be in the correct ratio.
Correct subscripts in a formula allow the sum of charges in the formula to
equal zero. All compound formulas must be neutral.
Steps to writing chemical formulas for ionic compounds
When the IUPAC name for an ionic compound is given, use the steps below
to write its correct formula.
Step 1: Write the correct ion symbols for the chemical name.
Use the Periodic Table to get the correct ion symbol for an element.
Use Table E to get the correct polyatomic ion symbol.
Always put parentheses around polyatomic atoms. Ex. (SO4)2Step 2: Criss-cross charge values so one becomes the subscript for the other.
Step 3: Clean up formula after criss-crossing by:
Reducing subscripts that are reducible to empirical form.
For polyatomic ion compounds:
Do not change subscripts of the polyatomic ion
Remove parentheses if subscript outside parentheses is a 1
Keep parentheses if subscript outside parentheses is greater than a 1
Steps shown above and on the next pages are to ensure that your final (correct)
formula has the correct element symbols and correct subscripts for the
compound name that is given. If you can write correct formulas without going
through all these steps, you should do so.
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Topic 5
Chemical Formulas and Equations
Writing Formulas for Ionic Compounds
Binary ionic compounds have formulas that are composed of two different
elements: a metal and a nonmetal. IUPAC names of binary compounds always
end with –ide. Examples: Calcium bromide, Aluminum sulfide, and Zinc oxide
Example 1: Calcium bromide
Step 1
Ca2+ Br 1–
Ca2+ Br1–
criss-crossing
charge values
Step 2
Ca1 Br2
Step 3
(formulas)
Aluminum Sulfide
Zinc Oxide
Al3+ S2–
Al3+ S2–
Zn2+ O2–
Zn2+ O2–
Al2 S3
Ca Br2
Zn2 O2
Al2 S3
ZnO
Ionic Compounds Containing a Polyatomic Ion
Polyatomic ions are composed of two or more atoms with an excess charge.
Reference Table E lists some common polyatomic ions . IUPAC names of polyatomic ions
typically end with -ate or –ite. Examples of compounds containing a polyatomic
ion are: sodium nitrate, calcium sulfite, and ammonium oxide.
Example 2: Sodium Nitrate
Step 1
Step 2
Step 3
(formulas)
+1
1–
Calcium Sulfite
Ammonium Oxide
2–
(NH4)1+ O2–
Na+1 (NO3)1–
Ca2+ (SO3)2–
(NH4)1+ O2–
Na1 (NO3)1
Ca2 (SO3)2
(NH4)2 O 1
NaNO3
CaSO3
(NH4)2 O
Na (NO3)
Ca
2+
(SO3)
Ionic Compounds Containing an Atom with Multiple Oxidation #s.
The stock system nomenclature uses Roman numerals in parentheses to
distinguish names of compounds produced by different positive oxidation states
of an atom. Examples of Stock system naming and the interpretation of a Roman
numeral in names are given below.
Iron (II) chloride
(II) indicates a compound of a +2 iron.
Iron (III) chloride
Nitrogen (IV) oxide
(III) indicates a compound of a +3 iron
(IV) indicates a compound of a +4 nitrogen
Example 3:
Iron (II) chloride
↓
Iron (III) chloride
↓
Fe2+ Cl1–
Fe3+ Cl1–
FeCl2
FeCl3
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Nitrogen (IV) oxide
↓
N4+ O2–
NO2
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Topic 5
Chemical Formulas and Equations
Writing IUPAC Names for Ionic Compounds
The IUPAC name is correctly written for a given formula when all of the
following are represented correctly.
. Atoms and/or ions in the formula are named correctly.
. Name ending , if necessary, is applied correctly.
. Roman numeral, if necessary, is correctly used.
Binary ionic compounds contain just two elements: a metal and nonmetal.
Examples are ZnCl2, CaO and Al2N3. IUPAC naming of binary compounds
involves changing the nonmetal ending to –ide. The metal name is never changed.
Example 5: ZnCl2
Zinc Chlorine
CaO
Calcium Oxygen
Al2 N3
Aluminum Nitrogen
Zinc Chloride
Calcium oxide
Aluminum nitride
Compounds containing a polyatomic ion generally contain three or more
elements. Examples are Mg2(PO4)3 , NH4NO3 and NH4Cl. When naming
compounds containing a polyatomic ion, no change should be made to the name
of the polyatomic ion (See Table E) or to the name of the metal.
Example 6: Mg2(PO4)3
Magnesium phosphate
NH4NO3
Ammonium nitrate
NH4Cl
Ammonium chloride
Compounds containing an element with multiple oxidation numbers
must be named using the Stock System. In each of the three formulas below,
the first element has multiple (+) oxidation numbers (See the Periodic Table to
confirm). A Roman numeral (in parentheses) is used in naming to identify which
positive charge of the element formed the compound.
Example 7: Sn F4
Tin (IV) fluoride
N2O
Nitrogen (I) oxide
Fe3(PO4)2
Iron (II ) phosphate
In some formulas (like the three above) the subscript of the second symbol in
each formula is used as the Roman Numeral (or the +charge) value in the ( ). In
some formulas (like the two below), the +charge value must be determined
mathematically by following steps given in the box to the left.
Example 8:
Assign (– ) charge value.
CrN2
MnSO4
3–
Cr N2
2 x 3- = - 6
Mn (SO4)2–
1 x 2- = -2
Multiply ( – ) charge by subscript to
get total (– ) in formula .
+6
+2
Determine total (+) needed to make
charges = 0
Use + charge as Roman numeral
Chromium (VI) nitride Manganese (II) sulfate
in ( ) to name the formula.
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Topic 5
Chemical Formulas and Equations
Writing Formulas and Naming Covalent (Molecular) Substances
Molecular (covalent) compounds are composed only of nonmetal atoms.
Binary molecular compounds, which contain two different nonmetals, are
commonly named with IUPAC recommended prefixes. A prefix in a chemical
name indicates how many of the nonmetal atom is in a given compound.
The table below lists prefixes for naming covalent compounds.
Number of atom
1
2
3
4
5
Prefix
monoditritetrapenta-
Number of atom
6
7
8
9
10
Prefix
hexaheptaoctanonadeca-
Writing formulas for covalent substances: Follow rules and examples below
. Prefixes are interpreted into subscripts for the elements.
. Absence of a prefix indicates that there is just one of that atom.
. No criss-crossing or reducing of formula into empirical form is necessary.
Example 9
Molecular name
Carbon dioxide
1C
Interpretation
2O
CO2
Formula
Carbon monoxide
1C
dinitrogen monoxide
1O
2N
CO
1O
N2O
Writing names for covalent substances: Follow rules and examples below.
. Subscripts are interpreted into prefixes for the elements
. No prefix is used when there is just one of the first nonmetal atom (see PCl3)
. The “a” or “o” of a prefix is dropped if the addition of the prefix resulted in a
name having two vowels next to each other (see N2O5)
In N2O5, the a in pentaoxide is dropped, and the formula is correctly named
pentoxide.
. Name ending for the second nonmetal atom must be changed to -ide
Example 10:
PCl3
1 Phosphorus 3 Chlorine
N2O5
2 nitrogen 5 oxygen
Phosphorus trichloride
dinitrogen pentoxide
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H2S
2 hydrogen 1 sulfur
dihydrogen monosulfide
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