Types of Formulas
The formulas for compounds can be expressed as an
empirical formula and as a molecular(true) formula.
Empirical
Molecular (true)
Name
CH
C2H2
acetylene
CH
C6H6
benzene
CO2
CO2
carbon dioxide
CH2O
C5H10O5
ribose
2
• An empirical formula represents the
simplest whole number ratio of the atoms
in a compound.
• The molecular formula is the true or
actual ratio of the atoms in a compound.
3
Formulas…
Structural Formulas tell
us-
Chemical Formulas tell us•Elements that make up the compound
•Shows the way the
atoms are joined
together in a molecule.
•Ratio of the elements in the compound
Example-
•Subscripts –below the element
•Tell you the number of
atoms
Example- Water-> H2O-> 2 hydrogen atoms
and one oxygen atom
Molecular Formulas tell us•Tell you the total number of atoms of
each element in 1 molecule of the
substance
Example- C2H2 ->Acetylene
-Carbon always forms 4
bonds
-Molecular Models show
orientation in 3
dimensions so are the
best molecular models
Empirical and Molecular Formulas
molar mass
simplest mass
=
a whole number = n
n = 1 molar mass = empirical mass
molecular formula = empirical formula
n = 2 molar mass = 2 x empirical mass
molecular formula =
2 x empirical formula
molecular formula = or > empirical formula
5
Empirical
Formula
Empirical
Mass
Timberlake LecturePLUS
Molecular
Formula
Molecular
Mass
6
The Empirical Formula
• The lowest whole number ratio of elements
in a compound.
• The molecular formula the actual ratio of
elements in a compound
• The two can be the same.
–
–
–
–
CH2 empirical formula
C2H4 molecular formula
C3H6 molecular formula
H2O both
Calculating Empirical
• Just find the lowest whole number ratio
– C6H12O6
– CH4N
• It is not just the ratio of atoms, it is also the
ratio of moles of atoms
• In 1 mole of CO2 there is 1 mole of carbon and
2 moles of oxygen
• In one molecule of CO2 there is 1 atom of C
and 2 atoms of O
Calculating Empirical
•
•
•
•
•
•
Pretend that you have a 100 gram sample of the
compound.
That is, change the % to grams.
Convert the grams to mols for each element.
Write the number of mols as a subscript in a
chemical formula.
Divide each number by the least number.
Multiply the result to get rid of any fractions.
Example
• Calculate the empirical formula of a compound
composed of 38.67 % C, 16.22 % H, and 45.11
%N.
• Assume 100 g so
• 38.67 g C x 1mol C = 3.220 mole C
12.01 gC
• 16.22 g H x 1mol H = 16.09 mole H
1.01 gH
• 45.11 g N x 1mol N = 3.219 mole N
14.01 gN
• 3.220 mole C
• 16.09 mole H
• 3.219 mole N
3.220 mol C and
3.219 mol N are
essentially the same.
C3.22H16.09N3.219
If we divide all of these by the smallest
one, it will give us the empirical formula
• The ratio is
• The ratio is
Example
3.220 mol C = 1 mol C
3.219 molN
1 mol N
16.09 mol H = 5 mol H
3.219 molN
1 mol N
• C1H5N1 is the empirical formula = CH5N
• A compound is 43.64 % P and 56.36 % O.
What is the empirical formula?
• 43.6 g P x
1mol P = 1.4 mole P
30.97 gP
• 56.36 g O x 1mol O = 3.5 mole O
16 gO
P1.4O3.5
Divide both by the lowest one
P1.4O3.5
• The ratio is 3.52 mol O = 2.5 mol O
1.42 mol P
1 mol P
P1O2.5
This
doesn’t
work!
• Multiply the result by 2 to get rid of any fractions.
2X
P1O2.5
= P2O5
Solve
• Caffeine is 49.48% C, 5.15% H, 28.87% N
and 16.49% O. What is its empirical
formula?
Empirical to Molecular
• Since the empirical formula is the lowest
ratio the actual molecule would weigh more
by a whole number multiple.
• Divide the actual molar mass by the mass of
one mole of the empirical formula.
Caffeine has a molar mass of 194 g. what is
its molecular formula? (You found
Caffeine’s empirical formula earlier, solve
for the empirical formula mass.)
Find x if
molar mass
x
empirical formula mass
194 g
97 g
2X
C4H5N2O1
C8H10N4O2.
=2
Example
• A compound is known to be composed of
71.65 % Cl, 24.27% C and 4.07% H. Its
molar mass is known (from gas density) is
known to be 98.96 g. What is its molecular
formula?
Example
1mol
71.65g Cl
35.5 g
1mol
24.27g C
12 g
1mol
4.07g H
1g
= 2.0mol
= 2.0mol
= 4.0mol
Cl2C2H4
Cl1C1H2
We divide by
lowest (2mol )
would give an empirical mass of
48.5g/mol
Its molar mass is known (from gas density)
is known to be 98.96 g. What is its molecular
formula?
molar mass
x
empirical formula mass
2 X Cl1C1H2
98.96 g
=2
48.5 g
= Cl2C2H4
Maegan
Lauren
Alex
Humpty Dumpty’s Downfall
Eggshells are
made of mostly
ionic compounds
such as calcium
phosphate,
Ca3(PO4)2, which
makes them brittle.
When broken,
eggshells break
into many pieces
that can’t be put
back together
again.
Formulas are the language used to give information about
chemicals. As a first step in studying this new language, you will
learn how to name and write formulas for ionic compounds.
Sodium chloride (NaCl) contains only sodium and chlorine.
Potassium iodide (KI) contains only potassium and iodine. Each is
an example of a binary compound, which is a compound that
contains only two elements.
Binary ionic compounds can have more than one ion of each
element, as in CaF2, but they are not composed of three or more
different elements.
To name binary compounds,:
First, write the name of the positively (+) charged ion, usually a
metal.
Second, add the name of the nonmetal or negatively (-) charged ion.
Third, change the end of the name to
-ide.
For example: NaCl
(+) is Na; (-) is Cl
1st: Na = Sodium
2nd: Cl = Chlorine
Write: Sodium Chlorine
Change: -ine to -ide  Chloride
Put them together: Sodium Chloride
Naming cations
Naming anions
Some oddballs: Nitrogen --> Nitride
Sulfur --> Sulfide
Try some:
AlCl3 ________________
___________________
ZnI2
________________
___________________
CaF2
________________
___________________
CaO
________________ ___________________
Al2O3 ________________
Al2O3
___________________
CaO
CaF2
Sodium chloride contains sodium ions that have a +1 charge and chloride
ions that have a -1 charge.
You have learned that compounds are electrically neutral. This means
that the sum of the charges in an ionic compound must always be zero.
One Na+ balances one Cl- in sodium chloride.
When you write a formula, you add subscripts to the symbols for the ions
until the algebraic sum of the ions’ charges is zero.
Example: Ca + F  [Ca]2+ + F- + F -  [Ca]2+ + 2F-  CaF2
Remember, the +’s and the -’s must cancel each other out.
If you have more than one ion of a given element in a compound, the
subscript indicates how many ions are present. The subscript is
predicted by the charges on the atoms.
Alumina is the common name for aluminum oxide. Aluminum is in
group 3, so it has 5 electrons and will lose 3 to become Al3+. Oxygen
has 6 valence electrons, so it will gain 2 electrons to become O2-.
Al
+
O
 Al3+ +
O
2-
Notice that one of aluminum’s 3 electrons has not been taken up by
oxygen. Because all the electrons must be accounted for, more than
one oxygen atom must be involved in the reaction. But oxygen cannot
going only one electron, so a second aluminum atom must be added to
give a second electron to oxygen. In all, two Al3+ ions must bond with
three O2- ions to form Al2O3. Remember that the charges in the
formula for aluminum oxide must add up to zero.
2Al3+ + 3O2-  Al2O3
Al + Al + O + O + O  Al3+ + Al3+ + O
2-
+ O
2-
+ O
2-
The easy way!
Write the formula for an ionic compound containing sodium and sulfur.
Analyze: Sodium is in Group 1, so it has an oxidation number of +1.
Sulfur is in group 6, so it has an oxidation number of -2.
Set up: Write the symbols for sodium and sulfur ions in formula form,
placing the positive ion first:Na+S2-
Solve: The formula as written has one positive charge and two negative
charges. To keep neutrality, one more positive charge is needed to
balance the 2- charge. This is done by adding a second sodium to the
formula. The correct formula is then written as Na2S.
Check: Check to be sure that you have not changed the charges of the
ions and that the overall charge of the formula is zero.
2(+1) + (-2) = 0 The formula is written correctly.
Try these:
Write the formula for each of the following compounds:
1. Lithium oxide
2. Calcium bromide
3. Sodium oxide
4. Aluminum sulfide
Try these:
Write the formula for the compound formed from each of the
following pairs of elements:
•
Barium and oxygen
•
Strontium and iodine
•
Lithium and chlorine
•
Radium and chlorine
Some ions contain more than one element.
These are called polyatomic ions. In a
polyatomic ion, a group of atoms is
covalently bonded. Although the
individual atoms have no charge, the
group as a whole has a charge.
The formulas and names of
some common polyatomic
ions are shown. Although
the charge is shown to the
right of the formula, it is the
whole ion, rather than the
last atom listed, that is
charged.
To name a compound containing a polyatomic ion, follow the same
rules as used in naming binary compounds. Name the positive ion
first, followed by the negative ion. However, do not change the
ending of the negative polyatomic ion.
Example: Calcium
Carbonate (Ca2+CO32-),
found in TUMS.
Try these:
1. Na2CrO4
_________________
__________________
2. KOH
_________________
__________________
3. Mg(NO3)
_________________
__________________
4. Ca(C2H3O2)
_________________
__________________
5. (NH4)2SO4
_________________
__________________
6. NH4Br
_________________
__________________
Write the formula for the compound that contains lithium and
carbonate ions.
Analyze: Lithium is in group 1, so it has a +1 charge. According to
the table on the previous slides, the carbonate ion has a -2 charge,
and its structure is CO32-.
Set up: Write the symbols for lithium carbonate in formula form:
Li+ CO32-
Solve: Decide the correct ratio of lithium ions to carbonate ions by
looking at their charges. In this case, the sum of the positive and
negative charges does not equal zero. Two lithium ions are needed
to balance the carbonate ion. Because you cannot change the
charges of the ions, you must ass a subscript of 2 to the Li+. The
correct formula for lithium carbonate is Li2CO3.
Check: Check to be sure the overall charge is zero:
2(1) + (-2) = 0 The formula is correct.
Writing the formulas for polyatomic ionic compounds is the same
as for binary ionic compounds.
Remember, if there is more than one polyatomic ion, use
parentheses () around it. If there is only one, no need for ().
Try these:
1. A compound containing ammonium and sulfite ions.
NH4+ x2
SO32+1
-2
x2
(NH4)2SO3
2. A compound containing barium and nitrate ions.
Ba
+2
Ba(NO3)2
NO3- x2
-1
x2
3. A compound containing calcium and phosphate ions.
4. Write the formula for sodium phosphate.
5. Write the formula for magnesium hydroxide.
6. Write the formula for potassium dichromate.
7. Write the formula for aluminum sulfate.
Transition elements form positive ions, just like other metals, but
most transition elements can form more than one type of positive
ion. In other words, transition elements can have more than one
oxidation number.
Copper (II) sulfate on
the left forms a blue
crystal. The oxidation
number is +2. Copper
(I) sulfate on the right
forms a black crystal.
The oxidation number
is +1.
Naming transition compounds is very easy. Follow the same
rules you learned for binary and polyatomic ions. Add in the
oxidation number between the 2 names in the form of Roman
numerals.
Element
Ion
Chemical Name
Chromium
Cr2+
Chromium (II)
Cr3+
Chromium (III)
Cr6+
Chromium (VI)
Co2+
Cobalt (II)
Co2+
Cobalt (III)
Cu+
Copper (I)
Cu2+
Copper (II)
Au+
Gold (I)
Au2+
Gold (III)
Fe2+
Iron (II)
Fe2+
Iron (III)
Mn2+
Manganese (II)
Mn2+
Manganese (III)
Mn7+
Manganese (VII)
Hg2+
Mercury (I)
Hg2+
Mercury (II)
Ni2+
Nickel (II)
Ni2+
Nickel (III)
Ni4+
Nickel (IV)
Cobalt
Copper
CrO
Cr2O3
Gold
Iron
Manganese
Mercury
CrO3
Nickel
Try these:
Write the names of the following compounds:
1. Pb(NO3)2
____________________________________________
2. Mn2O3
____________________________________________
3. Ni(C2H3O2)2
____________________________________________
4. HgF2
____________________________________________
5. CuBr3
_____________________________
6. Co3P2
_____________________________
7. AuI
_____________________________
8. WAt2
_____________________________
9. Pt2Ac3
_____________________________
Writing the formula for transition compounds:
Follow the same rules as you did for writing binary or
polyatomic ionic compound formulas. The only difference
is that you have to pay attention to the oxidation number.
Try these:
1. Copper (I) and sulfite
2. Tin (IV) and fluoride
3. Gold (III) and cyanide
4. Lead (II) and sulfide.
Naming covalent (molecular) compounds can seem very
complex…bur it is actually very systematic.
Substances are either organic (containing carbon) or inorganic
(not containing carbon).
Binary Inorganic Compounds contain 2 nonmetal elements
bonded covalently.
To name binary inorganic compounds, write out the name
of the first nonmetal and follow it with the name of the second
nonmetal with its ending changed to -ide.
How do you know which one is first? Which ever one is
farthest to the left on the periodic table.
If both elements are in the same group, the first element is
the one closer to the bottom. For example, in sulfur dioxide,
sulfur is named first because it is closer to the bottom of the table.
Remember, binary compounds, only contain 2 elements, but may
contain many atoms. Many different compounds can be formed by
the two same nonmetal elements. Binary inorganic compounds
have prefixes added to their names to help identify them.
one
mono-
Here are some basic prefixes you will
two
di-
see fairly often. Keep in mind, they
three trifour
tetra-
five
pent-
may go all the way up to 10.
Formula
Name
NO
nitrogen monoxide
NO2
nitrogen dioxide
N2O
dinitrogen monoxide
N2O5
dinitrogen pentoxide
Try naming these:
1. S2Cl2
2. CS2
3. SO3
4. P4O10
5. H2O
6. C2H6
7. SeI2
8. CH2F2
9. CO2
To write the formula for binary inorganic compounds for which
you are given the name, first write the symbols for each
element in the order they are given. Then add the appropriate
subscript for each prefix. Remember that the prefixes tell
how many atoms are present in the compound.
For example: sulfur hexaflouride
S
6
Try these:
1. Carbon tetrachloride
2. Iodine heptafluoride
3. Dinitrogen monoxide
4. Sulfur dioxide
F  SF6
Hydrates
For more lessons, visit
www.chalkbored.com
• Some compounds contain H2O in their structure. These compounds are called hydrates.
• This is different from (aq) because the H2O is
part of the molecule (not just surrounding it).
• The H2O can usually be removed if heated.
• A dot separates water: e.g. CuSO4•5H2O is
copper(II) sulfate pentahydrate.
• A greek prefix indicates the # of H2O groups.
Na2SO4•10H2O sodium sulfate decahydrate
nickel(II) sulfate hexahydrate
NiSO4•6H2O
sodium carbonate monohydrate Na2CO3•H2O
BaCl2•2H2O
barium chloride dihydrate
Naming Acids
Acids are divided into two groups: Binary and Oxyacids. Binary acids
consist of two elements. Oxyacids consist of 3 elements, one of which is
oxygen.
1. NAMING BINARY ACIDS: The name of the binary acid consists
of two words. The first word has three parts:
the “hydro” prefix
the root of the nonmetal element
the “ic” ending
The second word is always “acid”
Examples:
HCl = hydro chlor ic acid = hydrochloric acid
HBr = hydro brom ic acid = hydrobromic acid
HF = hydro fluor ic acid = hydrofluoric acid
2. NAMING OXYACIDS: These are more difficult to name because these
acids have hydrogen, a nonmetal, and may have varying numbers of oxygen
atoms. For example, H2SO5, H2SO4, H2SO3, and H2SO2 are all acids. How do
we name them? To begin, we need a point of reference. Our reference point is
this:
The “ate” ions (sulfate, nitrate, etc) make the “ic” acids (sulfuric acid, nitric acid)
Examples:
SO42- = sulfate ion H2SO4 = sulfuric acid
NO3- = nitrate ion HNO3 = nitric acid
Once we have our point of reference, the acid with one more oxygen than the ic acid is called the per-_________-ic acid. The acid with one less oxygen then
the -ic acid is called the ___________-ous acid. If the acid has one less oxygen
than the -ous acid, it is called the hypo-____________-ous acid.
Examples:
H2SO5 = persulfuric acid
HNO4 = pernitric acid
H2SO4 = sulfuric acid
HNO3 = nitric acid
H2SO3 = sulfurous acid
HNO2 = nitrous acid
H2SO2 = hyposulfurous acid
HNO = hyponitrous acid
Some compounds have common names. For example, NaCl is salt.
Would you go into a restaurant and order a glass of dihydrogen
monoxide? Probably not. You would order water.
Acids and bases are also known by common names, rather than by their
chemical names. Some examples are below:
Formula
Name
HCl
Hydrochloric Acid
H2SO4
Sulfuric Acid
H3PO4
Phosphoric Acid
HNO3
Nitric Acid
HC2H3O2
Acetic Acid
NaOH
Sodium Hydroxide (base)
KOH
Potassium Hydroxide (base)
NH3
Ammonia (base)
Naming Alkanes
1.Naming an organic compound requires several steps. What follows is a minimal
outline. Consult your textbook for more details and examples. Find the longest
continuous chain of carbon atoms. This longest chain may not be written in a straight
line.
2.Name that longest chain with the appropriate alkane name.
3.Find the substituents and name each of them with the appropriate group name.
4.Number the carbons in the chain such that the lowest number is given to the group
nearest one end of the chain.
5.List the groups alphabetically in front of the longest chain name.
6.Precede each group name with a prefix indicating how many of them are present in
the molecule. In front of the prefix, list the numbers of each group in the molecule and
separate the numbers with a hyphen.