CIE IGCSE Tutorial
CHEMISTRY
(0625)
STOICHIOMETRY AND MOLE CONCEPT
Stoichiometry
Stoichiometry is a section of chemistry that involves using relationships between reactants and/or
products in a chemical reaction to determine desired quantitative data and design chemical
formula in according to its combining powers.
Writing chemical formula (molecular formula)
The formula of a compound tells us the number of each type of atom in a molecule. The formula of
sulphuric acid is H2SO4. This tells us that there are two moles of hydrogen, one mole of sulphur and four
moles of oxygen. For ionic compounds, the formula can be worked out by making sure the total charge
equals zero. The valence can also be used to determine a chemical formula. The valence represents the
total numbers of covalent bonds an atom can form, or the charge on an ion. Valencies can be worked
out by using the periodic table:
Naming Covalent Compounds
Many of these compounds have common names such as "bromine", "ammonia" and " carbon dioxide ". However,
simple covalent compounds are generally named by using prefixes to indicate how many atoms of each element
are shown in the formula. Also, the ending of the last (most negative) element is changed to -ide.
The prefixes used are mono-, di-, tri-, tetra-, penta-
, hexa-, and so forth. The mono- prefix is usually not
used for the first element in the formula. The "o" and Number of bonds Prefix
"a" endings of these prefixes commonly are dropped
1 mono-
when they are attached to "oxide.”.
2 di-
E.g. Carbon dioxide – CO2
3 tri-
Sulphur trioxide – SO3 4 tetra-
5 penta-
Phophorous pentachloride Pcl5
6 hexa-
Carbon tetrachloride CCl4
Naming ionic compounds
,For an ionic compound there is a rule for finding the correct formula. In every ionic formula the cation is written
first and the anion written second. In the formula, the charge on one becomes the subscripts of other.
Before you write the chemical formula for ionic compounds, you should know the valence shell electron
number and the ionic charge of anions (negative ions) and cations (positive ions). You can learn the ionic
charge of anions and cations referring periodic table of elements. But polyatomic anions and cations are
not given in periodic table and you have to memorise them using polyatomic ion charge sheet.
Group I II III IV V VI VII VIII
Valency 1+ 2+ 3+ 4+ or 4- 3- 2- 1- 0
(ionic charge)
Combining power of metal and non metal
GROUP I II III IV V VI VII VIII
NUMBER
COMBINING 1 2 3 4 3 2 1 0
POWER( how lose lose lose lose or gain gain gain
many gain
electrons can
lose or gain)
Examples:
Here are the steps you use to work out the formula of magnesium oxide:
, 1. Write down the name of the compound (magnesium oxide)
2. Write down the chemical symbols for the elements in the compound (Mg and
O)
3. Use the Periodic Table to find the ‘combining power’ of each element from the
table below. Write the combining power of each element under its symbol (Mg
2, O 2)
4. If the numbers can be cancelled, cancel them (Mg O)
5. Swap over the combining powers and write down the symbols
What about Calcium Chloride?
1. Write down the name of the compound (Calcium Chloride)
2. Write down the chemical symbols for the elements in the compound (Ca and
Cl)
3. Use the Periodic Table to find the combining power of each element. Write
down the combining power for each element under its symbol (Ca1 Cl2)
4. If the numbers can be cancelled down, cancel them (they can’t be)
5. Swap over the combining powers. Write them below the symbols as a subscript.
6. If the number is one – please don’t write it!
The correct formula of calcium chloride is CaCl2
This is a table of all the POSITIVE METAL IONS that you should know in your syllabus.
Valency 1 Valency 2 Valency 3
lithium Li+ magnesium Mg2+ aluminium Al3+
sodium Na+ calcium Ca2+ iron III Fe3+
potassium K+ strontium Sr2+ chromium Cr3+
silver Ag+ barium Ba2+
hydronium H3O+ copper II Cu2+
(or hydrogen) H+ lead II Pb2+
+
ammonium NH4 zinc Zn2+
copper I Cu+ manganese II Mn2+
mercury I Hg+ iron II Fe2+
tin II Sn2+
, This is a table of all the NEGATIVE NON-METAL IONS
Valency 1- Valency 2- Valency 3-
3-
fluoride F- oxide O2- phosphate PO4
chloride Cl- sulphide S2- Nitride N-3
2-
bromide Br - carbonate CO3
2-
iodide I- sulphate SO4
2-
hydroxide OH- sulphite SO3
- -
nitrate NO3 dichromate Cr2O7
- 2-
bicarbonate HCO3 chromate CrO4
- 2-
bisulphate HSO4 oxalate C2O4
- 2-
nitrite NO2 thiosulphate S2O3
- 2-
chlorate ClO3 tetrathionate S4O6
MnO 4- monohydrogen HPO 2-4
permanganate
phosphate
hypochlorite OCl-
-
Dihydrogenphosphate H2PO4
Ethanoate/ Acetate
CH3COO-
Groups I to IV – group number is same as combining power
Group V to VIII – combining power = 8 – group number
If an element is not in groups 1 to 8 it will be a transition metal element. Some transition
elements have multiple valencies and so multiple combining powers. It shows by writing in
Roman numeral in brackets.
Examples:
Iron (II) sulphate = FeSo4
Iron (III) sulphate = Fe(So4)3
Phosphorus is in group 5 so you would expect it to have a combining power of 3.
But it can form a compound called phosphorus (V) oxide in which it has the combining
power of 5.
Write the chemical formula for the followings
CHEMISTRY
(0625)
STOICHIOMETRY AND MOLE CONCEPT
Stoichiometry
Stoichiometry is a section of chemistry that involves using relationships between reactants and/or
products in a chemical reaction to determine desired quantitative data and design chemical
formula in according to its combining powers.
Writing chemical formula (molecular formula)
The formula of a compound tells us the number of each type of atom in a molecule. The formula of
sulphuric acid is H2SO4. This tells us that there are two moles of hydrogen, one mole of sulphur and four
moles of oxygen. For ionic compounds, the formula can be worked out by making sure the total charge
equals zero. The valence can also be used to determine a chemical formula. The valence represents the
total numbers of covalent bonds an atom can form, or the charge on an ion. Valencies can be worked
out by using the periodic table:
Naming Covalent Compounds
Many of these compounds have common names such as "bromine", "ammonia" and " carbon dioxide ". However,
simple covalent compounds are generally named by using prefixes to indicate how many atoms of each element
are shown in the formula. Also, the ending of the last (most negative) element is changed to -ide.
The prefixes used are mono-, di-, tri-, tetra-, penta-
, hexa-, and so forth. The mono- prefix is usually not
used for the first element in the formula. The "o" and Number of bonds Prefix
"a" endings of these prefixes commonly are dropped
1 mono-
when they are attached to "oxide.”.
2 di-
E.g. Carbon dioxide – CO2
3 tri-
Sulphur trioxide – SO3 4 tetra-
5 penta-
Phophorous pentachloride Pcl5
6 hexa-
Carbon tetrachloride CCl4
Naming ionic compounds
,For an ionic compound there is a rule for finding the correct formula. In every ionic formula the cation is written
first and the anion written second. In the formula, the charge on one becomes the subscripts of other.
Before you write the chemical formula for ionic compounds, you should know the valence shell electron
number and the ionic charge of anions (negative ions) and cations (positive ions). You can learn the ionic
charge of anions and cations referring periodic table of elements. But polyatomic anions and cations are
not given in periodic table and you have to memorise them using polyatomic ion charge sheet.
Group I II III IV V VI VII VIII
Valency 1+ 2+ 3+ 4+ or 4- 3- 2- 1- 0
(ionic charge)
Combining power of metal and non metal
GROUP I II III IV V VI VII VIII
NUMBER
COMBINING 1 2 3 4 3 2 1 0
POWER( how lose lose lose lose or gain gain gain
many gain
electrons can
lose or gain)
Examples:
Here are the steps you use to work out the formula of magnesium oxide:
, 1. Write down the name of the compound (magnesium oxide)
2. Write down the chemical symbols for the elements in the compound (Mg and
O)
3. Use the Periodic Table to find the ‘combining power’ of each element from the
table below. Write the combining power of each element under its symbol (Mg
2, O 2)
4. If the numbers can be cancelled, cancel them (Mg O)
5. Swap over the combining powers and write down the symbols
What about Calcium Chloride?
1. Write down the name of the compound (Calcium Chloride)
2. Write down the chemical symbols for the elements in the compound (Ca and
Cl)
3. Use the Periodic Table to find the combining power of each element. Write
down the combining power for each element under its symbol (Ca1 Cl2)
4. If the numbers can be cancelled down, cancel them (they can’t be)
5. Swap over the combining powers. Write them below the symbols as a subscript.
6. If the number is one – please don’t write it!
The correct formula of calcium chloride is CaCl2
This is a table of all the POSITIVE METAL IONS that you should know in your syllabus.
Valency 1 Valency 2 Valency 3
lithium Li+ magnesium Mg2+ aluminium Al3+
sodium Na+ calcium Ca2+ iron III Fe3+
potassium K+ strontium Sr2+ chromium Cr3+
silver Ag+ barium Ba2+
hydronium H3O+ copper II Cu2+
(or hydrogen) H+ lead II Pb2+
+
ammonium NH4 zinc Zn2+
copper I Cu+ manganese II Mn2+
mercury I Hg+ iron II Fe2+
tin II Sn2+
, This is a table of all the NEGATIVE NON-METAL IONS
Valency 1- Valency 2- Valency 3-
3-
fluoride F- oxide O2- phosphate PO4
chloride Cl- sulphide S2- Nitride N-3
2-
bromide Br - carbonate CO3
2-
iodide I- sulphate SO4
2-
hydroxide OH- sulphite SO3
- -
nitrate NO3 dichromate Cr2O7
- 2-
bicarbonate HCO3 chromate CrO4
- 2-
bisulphate HSO4 oxalate C2O4
- 2-
nitrite NO2 thiosulphate S2O3
- 2-
chlorate ClO3 tetrathionate S4O6
MnO 4- monohydrogen HPO 2-4
permanganate
phosphate
hypochlorite OCl-
-
Dihydrogenphosphate H2PO4
Ethanoate/ Acetate
CH3COO-
Groups I to IV – group number is same as combining power
Group V to VIII – combining power = 8 – group number
If an element is not in groups 1 to 8 it will be a transition metal element. Some transition
elements have multiple valencies and so multiple combining powers. It shows by writing in
Roman numeral in brackets.
Examples:
Iron (II) sulphate = FeSo4
Iron (III) sulphate = Fe(So4)3
Phosphorus is in group 5 so you would expect it to have a combining power of 3.
But it can form a compound called phosphorus (V) oxide in which it has the combining
power of 5.
Write the chemical formula for the followings
