Chapter 7 Notes
Chemical Formulas
&
Chemical Compounds
West Valley High School
General Chemistry
Mr. Mata
Standard 3d
• Determine the molar mass of
a molecule from its chemical
formula and atomic masses.
Essential Question
• How do you write chemical
formulas for chemical
compounds using a set of
standard rules?
Ions
• Ions: atom (groups of atoms = polyatomic ions)
that have a charge.
• Number of protons (p+) is not equal to the number
of electrons (e-).
p + ≠ e-
• Cation: A positive ion
• Mg2+, NH41+
• Anion: A negative ion
• Cl 1 -, SO42 -
• Ionic Bonding: Force of attraction between
oppositely charged ions.
Predicting Ionic Charges
Group 1: Lose 1 e- to form +1 ions
H+1 Li+1 Na+1 K+1
Predicting Ionic Charges
Group 2: Loses 2 e-’s to form +2 ions
Be+2
Mg+2
Ca+2
Sr+2
Ba+2
Predicting Ionic Charges
B+3
Al+3
Ga+3
Group 13: Loses 3
e-’s to form
+3 ions
Predicting Ionic Charges
Neither! Group 14
elements rarely form
ions.
Group 14: Lose 4
e-’s or gain
4 e-’s?
Predicting Ionic Charges
N-3 Nitride
P-3 Phosphide
As-3 Arsenide
Group 15: Gains 3
e-’s to form
-3 ions
Predicting Ionic Charges
O-2 Oxide
S-2 Sulfide
Se-2 Selenide
Group 16: Gains 2
e-’s to form
-2 ions
Predicting Ionic Charges
F-1 Fluoride
Br-1 Bromide
Cl-1Chloride
I-1 Iodide
Group 17: Gains 1
e- to form
-1 ions
Predicting Ionic Charges
Group 18: Stable
Noble gases do not
form ions!
Predicting Ionic Charges
Groups 3 - 12: Many transition elements
have more than one possible ionic charge.
Iron(II) = Fe+2
Iron(III) = Fe+3
Predicting Ionic Charges
Groups 3 - 12: Some transition elements
have only one possible ionic charge.
Zinc (II) = Zn+2
Silver (I) = Ag+1
Writing Ionic Compound Formulas
Example: Magnesium carbonate
1. Write the formulas for the cation
and anion, including CHARGES!
2. Check to see if charges
are balanced.
Mg+2 CO3-2
They are balanced!
MgCO3
Writing Ionic Compound Formulas
Example: Iron(III) chloride
1. Write the formulas for the cation
and anion, including CHARGES!
2. Check to see if charges
are balanced.
3. Balance charges , if necessary,
using subscripts.
Fe+3 Cl-1
3
Not balanced!
FeCl3
Writing Ionic Compound Formulas
Example: Barium nitrate
1. Write the formulas for the cation
and anion, including CHARGES!
2. Check to see if charges are
balanced.
+2
(
Ba NO3 ) 2
3. Balance charges , if necessary,
using subscripts. Use parentheses
if you need more than one of a
polyatomic ion.
Not balanced!
Ba(NO3)2
Writing Ionic Compound Formulas
Example: Ammonium sulfate
1. Write the formulas for the cation
and anion, including CHARGES!
2. Check to see if charges
are balanced.
( NH4+) SO4-2
2
3. Balance charges , if necessary,
using subscripts. Use parentheses
if you need more than one of a
polyatomic ion.
Not balanced!
(NH4)2SO4
Writing Ionic Compound Formulas
Example: Aluminum sulfide
1. Write the formulas for the cation
and anion, including CHARGES!
+3
Al
2. Check to see if charges
are balanced.
2
3. Balance charges , if necessary,
using subscripts.
-2
S
3
Not balanced!
Al2S3
Writing Ionic Compound Formulas
Example: Zinc hydroxide
1. Write the formulas for the cation
and anion, including CHARGES!
2. Check to see if charges are
balanced.
+2
Zn
3. Balance charges , if necessary,
using subscripts. Use parentheses
if you need more than one of a
polyatomic ion.
( OH- )2
Not balanced!
Zn(OH)2
Writing Ionic Compound Formulas
Example: Aluminum phosphate
1. Write the formulas for the cation
and anion, including CHARGES!
2. Check to see if charges are
balanced.
3+
Al
PO4
3-
They ARE balanced!
AlPO4
TOP SECRET!!!
• DO NOT REVEAL
THIS SECRET
TECHNIQUE TO
ANY OTHER
CHEMISTRY CLASS!!!
Criss-Cross Shortcut Method
Ex: Na+1 S2Na
2
Ex: Mg2+
Mg1
S
1
= Na2S
sodium sulfide
Br-1
Br2 = MgBr2
magnesium bromide
Criss-Cross Shortcut Method
Example: Al
3+
S
2-
Al2
S3 =
Al2S3 aluminum sulfide
Example: Al
3+
CO3 2-
Al2
(CO3)3 =
Al2(CO3)3 aluminum carbonate
Naming Ionic Compounds
• 1. Cation first, then anion
• 2. Monatomic cation = name of element
• Ca2+ = calcium ion
• 3. Monatomic anion = root + -ide
• Cl -1 = chloride
• CaCl2 = calcium chloride
Naming Ionic Compounds
(continued)
Metals with multiple oxidation states
some metals form more than one cation.
• - use Roman numeral in name.
• -
• PbCl2
• Pb2+ is cation
• PbCl2 = lead(II) chloride
Naming Binary Compounds
•
-
•
•
•
•
-
Compounds between two nonmetals.
First element in formula is named first.
Second element named as if it were an anion.
Use prefixes.
Only use mono on second element P2O5
CO2
CO
N2O
= diphosphorus pentoxide
= carbon dioxide
= carbon monoxide
= dinitrogen monoxide
Calculating Formula Mass
Calculate the formula mass of magnesium carbonate,
MgCO3.
24.31 g + 12.01 g + 3(16.00 g) = 84.32 g
Calculating Percentage Composition
Calculate the percentage composition of magnesium
carbonate, MgCO3.
From previous slide:
24.31 g + 12.01 g + 3(16.00 g) = 84.32 g
 24.31 
Mg  
  100  28.83%
 84.32 
 12.01 
C 
  100  14.24%
 84.32 
 48.00 
O
  100  56.93%
 84.32 
100 %
Empirical formula: lowest whole number ratio
of atoms in a compound.
Molecular formula: true number of atoms of
each element in the formula of a compound.
Molecular:
H2O
C6H12O6
C12H22O11
Empirical:
H2O
CH2O
C12H22O11
Formulas for ionic compounds are ALWAYS
empirical (lowest whole number ratio).
Examples:
NaCl
MgCl2
Al2(SO4)3
K2CO3
Empirical Formula Determination
1. Base calculation on 100 grams of compound.
2. Determine moles of each element in 100
grams of compound.
3. Divide each value of moles by the smallest of
the values.
4. Multiply each number by an integer to obtain
all whole numbers.
Empirical Formula Determination
Adipic acid contains 49.32% C, 43.84% O, and
6.85% H by mass. What is the empirical formula
of adipic acid?
 49.32 g C 1 mol C   4.107 mol C
12.01 g C 
 6.85 g H 1 mol H   6.78 mol H
1.01 g H 
 43.84 g O 1 mol O   2.74 mol O
16.00 g O 
Empirical Formula Determination
(part 2)
Divide each value of moles by the smallest of the
values.
4.107
mol
C
Carbon:
 1.50
2.74 mol O
6.78 mol H
Hydrogen:
 2.47
2.74 mol O
2.74 mol O
Oxygen:
 1.00
2.74 mol O
Empirical Formula Determination
(part 3)
Multiply each number by an integer to obtain all
whole numbers.
Carbon: 1.50
x 2
3
Hydrogen: 2.50
x 2
5
Oxygen: 1.00
x 2
2
Empirical formula: C3H5O2
Chapter 7 SUTW Prompt
• Describe the differences between
writing chemical formulas for molecular
compounds and ionic compounds.
• Complete an 8-10 sentence paragraph
using the SUTW paragraph format.
Hilight using green, yellow, and pink.
• Due Date: Tomorrow (start of class).