Chapter four
Stoichiometry
Symbols and formula:
The symbol of an element is one or 2 letters (1st is capital and the 2nd is small)
ex: calcium Ca or potassium K or sodium Na
Important notes:
1. The large number in front of the symbol, means separate atoms or
molecules or compounds.
Ex: 2MgO = MgO + MgO
2. The number below the symbol means that atoms are bonded together.
Ex:
O2 means O=O , MgCl2 means Mg2+ +2ClElement
symbol
valency
1.
Carbon
C
4
2.
Nitrogen
N
3
3.
Silver
Ag
1
4.
Copper
Cu
2 or 1
5.
Iron
Fe
2 or 3
6.
Lead
Pb
2 or 4
7.
Zinc
Zn
2
8.
Potassium
K
1
9.
Calcium
Ca
2
10.
Oxygen
O
2
11.
Sulfur
S
2
For group of atoms having charge ( radicals):
Charge
Radical
Formula
+1
▪ Ammonium
▪ NH4+
-1
▪ Hydroxide
• OH-
▪ Nitrate
• NO3-
▪ Nitrite
• NO2-
▪ Hydrogen
• HCO3-
carbonate
-2
3-
▪ Hydrogen sulfate
• HSO4-
➢ Sulfate
➢ SO42-
➢ Sulfite
➢ SO32-
➢ Carbonate
➢ CO32-
✓ Phosphate
✓ PO43-
✓ Phosphite
✓ PO33-
Naming compounds:
1.
ide: compound containing 2 different elements only (except
~~~~~~~~``~
hydroxide) ex: calcium chloride CaCl2 or potassium iodide KI
2.
~~~~~~~ate
: compound containing plenty of oxygen ex: potassium sulfate
K2SO4
3.
~~~~~~~ite
: compound containing one oxygen less than ~~~~ate ex:
potassium sulfite K2SO3
Working out the formula of a compound:
1. Write the symbols of the element/ radical with their valency.
2. Divide by the common denominator if present.
3. Cris-cross
Ex: write the formula of :
i.
ii.
Magnesium oxide:
common number is 2 so  by 2
Mg 2
O2
Mg 2/2
O2 /2

The symbol is MgO
Potassium sulfate:
K1
SO42-
cris-cross
K2SO4
iii.
Iron (II)chloride:
Fe 2
Cl1
cris-cross
FeCl2
iv.
Copper (II)iodide:
Cu2
I1
CuI2
v.
Copper(I) iodide:
cris-cross
Cu1
I1
cris-cross
CuI
❖ Note:
If a radical is involved and a number is to be put after it , a bracket is
put around the radical.
Examples:
•
Aluminium sulfate:
Al3+
SO42- cris-cross
Al2(SO4)3
•
Ammonium phosphate:
NH41+
PO43-
(NH4)3PO4
➢ Naming formula:
▪ FeO:
iron(II)oxide
▪ NaNO3:
sodium nitrate
▪ FeSO4:
iron(II)sulphate
▪ ZnCO3:
zinc carbonate
▪ Al(NO3)3:
aluminium nitrate
▪ (NH4)2 SO4:
Ammonium sulfate
▪ Na2S:
sodium sulfide
▪ Mg3P2:
magnesium phosphide
➢ Learn the following formula:
▪ H2O  water
▪ HCl (aq)  hydrochloric acid
▪ HCl(g)  hydrogen chloride
▪ H2SO4(aq)  sulfuric acid
▪ NO2(g)  nitrogen dioxide
▪ NO(g)  nitrogen monoxide
▪ HNO3(aq)  nitric acid
▪ CO2(g)  carbon dioxide
▪ CO (g) carbon monoxide
▪ SO3  sulfur trioxide
▪ SO2(g)  sulfur dioxide
▪ H2O2(l)  hydrogen peroxide
▪ NH3(g)  ammonia gas
▪ NH4OH(aq)  aqueous ammonia
Note:
Most of the common gases are diatomic molecules, ex: Cl2 , F2 , O2 , N2 ,H2
also Br2(l) and I2(s).
Balancing equations:
1.
Write the formulae of each substance in the word equation.
Ex: Iron + hydrochloric acid → iron (II ) chloride + hydrogen gas
Fe
2.
+ HCl
→ FeCl2
+ H2
Check that the number of atoms of each element on R.H.S equals
the number of atoms on the L.H.S.
L.H.S
R.H.S
Fe :
1
1
H:
1 *2
2
Cl:
1 *2
2
 we must put 2 in front of HCl (multiply by 2).
Very important note:
Never alter the number within a formula.
Example:
Lead(IV) oxide
Pb4/2 O2/2
→ lead(II) oxide + oxygen
Pb2/2O2/2
+ O2
→ PbO
PbO2
+ O2
Then we balance each element in the equation.
L.H.S
R.H.S
Pb:
1
1
O:
2
3
The PbO is causing the odd no. in R.H.S , SO multiply it by 2 the balance
the equation.
2PbO2
•
→ 2 PbO
+ O2
State symbols:
S → solid , l→ liquid , aq →aqueous dissolved in water/ solution in water.
Ex:
Na2CO3(s) + 2HCl(aq) → 2 NaCl(aq) + H2O(l) +CO2(g)
Deduce the formula of compound from a model: