Name: _________________________________
Practice Problems - Determining Oxidation Numbers
1. Determine the oxidation number of each element in the following compounds.
Oxidation Numbers for each Element
a.
SnCl4
Sn
Cl
b.
Ca3P2
Ca
P
c.
SnO
Sn
O
d.
Ag2S
Ag
S
e.
HI
H
I
f.
N2H4
N
H
g.
Al2O3
Al
O
h.
S8
S
i.
HNO2
H
j.
O2
O
k.
H3O+
H
O
l.
ClO3-
Cl
O
m. S2O32-
S
O
n.
KMnO4
K
Mn
O
o.
(NH4)2SO4
N
H
S
N
O
O
2. Determine the oxidation number of carbon in each of the following compounds:
a. methane, CH4
b. formaldehyde, CH2O
c. carbon monoxide, CO
d. carbon dioxide, CO2
Redox Reactions - Practice Problems - Determining Oxidation Numbers - KEY
Source: http://www.saskschools.ca/curr_content/chem30_05/6_redox/practice/redox_practice1.htm
1. Determine the oxidation number of each element in the following compounds.
Oxidation Numbers for each Element
a.
SnCl4
Sn
+4
Cl
-1
There are 4 Cl atoms
b.
Ca3P2
Ca
+2
P
-3
c.
SnO
Sn
+2
O
-2
d.
Ag2S
Ag
+1
S
-2
e.
HI
H
+1
I
-1
f.
N2H4
N
-2
H
+1
g.
Al2O3
Al
+3
O
-2
h.
S8
S
0
i.
HNO2
H
+1
j.
O2
O
0
k.
H3O+
H
+1
O
-2
This is an ion – the
final charge is 1+
l.
ClO3-
Cl
+5
O
-2
This is an ion too –
charge is 1-
m. S2O32-
S
+2
O
-2
n.
KMnO4
K
+1
Mn
+7
O
-2
o.
(NH4)2SO4
N
-3
H
+1
S
+6
Think valence
electrons – how many
does sulfur need?
watch the sign for N!
Pure element
N
+3
O
-2
pure element!
O
-2
Extra help for KMnO4 & (NH4)2SO4
You will need to recognize polyatomic ions. It will simplify determining oxidation numbers
to break molecules with polyatomic ions into two separate ions, then find oxidation
numbers for each part separately.
KMnO4
K+
Oxidation Number =
MnO4-
oxygen’s oxidation number = -2
sum of ox. nos. for MnO4therefore oxidation number of
Mn
4 oxygens total =
Hydrogen’s oxidation number =
+1
charge of ammonium ion
4 hydrogens total
=
(NH4)2SO4 NH4+
+1
-8
-1
+7
+4
+1
therefore oxidation number of N
-3
+
Note – there are two NH4 ions, but the oxidation numbers in both will be
the same!
SO42-
oxygen’s oxidation number = -2
charge of sulfate ion
4 oxygens total =
-8
-2
therefore oxidation number of S
+6
2. Determine the oxidation number of carbon in each of the following compounds:
a. methane, CH4
C = -4
element
H
C
Ox.
No.
+1
-4
b. formaldehyde, CH2O
C=0
No.
Total
Atoms
4
+4
1
-4
SUM
0
c. carbon monoxide, CO
C = +2
element
O
C
Ox.
No.
-2
+2
No.
Total
Atoms
1
-2
1
+2
SUM
0
element
H
O
C
Ox.
No.
+1
-2
0
No.
Atoms
2
1
1
SUM
Total
+2
-2
0
0
d. carbon dioxide, CO2
C = +4
element
O
C
Ox.
No.
-2
+4
No.
Atoms
2
1
SUM
Total
-4
+4
0