How to assign oxidation numbers:
You have been doing this since you have been naming compounds. When you make the total be zero,
you have to know most of the oxidation numbers. The exception is the positive part of the polyatomic
ions.
SO some rules to know:
1. The sum of the oxidation numbers of all the elements of a COMPOUND ADD up to = zero.
2. The sum of the oxidation numbers of all the elements of a POLYATOMIC ION (page 2 of the pink sheet)
= Its charge.
3. The oxidation number for OXYGEN is ALWAYS -2. (Except for peroxide. RARE)
4. The oxidation number for HYDROGEN is ALWAYS +1. (99% of the time.)
5. Elements that are not part of a compound or polyatomic ion have a zero oxidation number.
How to:
Example 1:
Find the oxidation number of C in CO2.
The oxidation number of oxygen is always -2. So:
C + 2 oxygen ions with a -2 charge each = 0
C + 2(-2) = 0
C-4=0
C = +4
When you have a compound you can work out the oxidation numbers in two different
ways. You can see these demonstrated below.
NaClO3 What is the oxidation number of Cl?
Na = +1 (see Rule #6)
Cl = ?
O = -2 (always)
Al2(SO4)3
Compound method
Na + Cl + 3(O) = 0
+1+ Cl + 3(-2) = 0
+1 + Cl -6 = 0
Cl - 5 = 0
Cl = +5
Polyatomic Ion method
ClO3- is the polyatomic ion
of the compound
Cl + 3(O) = -1
Cl + 3(-2) = -1
Cl – 6 = -1
Cl = -1 + 6
Cl = +5
Pull out one polyatomic ion SO4-2 is the polyatomic ion
2A1 + 3S + 12 O = 0
2(+3) + 3S + 12(-2) = 0
+6 +3S - 24 = 0
3S - l8 = 0
3S = 0 + 18
3
3
S = +6
S + 4(O) = -2
S + 4(-2) = -2
S - 8 = -2
S = -2 + 8
S = +6
Formula Mass:
the sum of all the atomic masses in a formula.
To make these calculations easier, round all atomic mass numbers to the nearest hundredth.
Some answers on your homework—Check your answers. (I hope these are correct, but I did them quickly and may
have an error. Don’t fret. I will answer your questions tomorrow.) You will turn in your tests tomorrow.
How to do formula mass, gmm, and molar mass?
(all three of these names mean the same thing)
Find the gmm of Fe2O3
1. Use the periodic table to find the atomic mass (decimal number) and round off to the nearest 100th.
Fe – 55.85
O - 16.00
2. Now, multiple the number of atoms of each element times the atomic mass.
Fe – 2(55.85) = 111.70
O - 3(16.00) = 48.00
3. Now, add these numbers together and add the unit ―g‖.
Fe – 2(55.85) = 111.70
O - 3(16.00) = 48.00
159.70 g
Example 2:
Find the gmm of Al2(SO4)3
4. Use the periodic table to find the atomic mass (decimal number) and round off to the nearest 100th.
Al – 26.98
S - 32.07
O - 16.00
5. Now, multiple the number of atoms of each element times the atomic mass.
Al – 2(26.98) = 53.96
Because of the Parentheses () in the compound Al(SO4)3
S - 3(32.07) = 96.21
you have to multiply the subscripts on SO4 times 3.
O - 12(16.00) = 192.00
Now, add these numbers together and add the unit ―g‖.
Al – 2(26.98) = 53.96
S - 3(32.07) = 96.21
O - 12(16.00) = 192.00
342.17 g