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Formula
If you don’t understand this section you will not understand chemistry, get help if
required.
Rule 1
For most elements the correct ‘formula’ is the chemical symbol, but there are a few
elements that exist as diatomic elements.
1. Answer the following:
What is a diatomic element?
Name the 7 diatomic elements.
2. Write the chemical formula for the following elements.
a. sodium
b. lithium
c. potassium
d. calcium
e. hydrogen
f. chlorine
g. helium
h. nitrogen
i. copper
j. oxygen
Rule 2 Calculating formulae for two element compounds using prefixes
When using prefixes to obtain formula there is no simplifying required.
Copy and complete the following table
Prefix
Meaning
Compound
Formula
Mono
1
Carbon monoxide
CO
Di
Sulphur dioxide
Tri
Sulphur trioxide
tetra
Carbon tetrachloride
Penta
Divanadium pentoxide
Hexa
Uranium hexafluoride
3. Now copy and complete the following examples
Name of compound
formula
Name of compound
a) nitrogen monoxide
e) tetraarsenic hexaoxide
b) diphosphorus trioxide
f) nitrogen dioxide
c) silicon tetrachloride
g) disulphur dichloride
d) boron trifluoride
formula
Rule 3 –
For compounds with names ending in ‘ide’ (2 elements only) and no prefixes you have to use
the valencies to work out the formula.
The valency of an element can be worked out from its position in the periodic table.
Group number
1
2
3
4
5
6
7
Valency
1
2
3
4
3
2
1
Use the 5 step method
Step 1 – write down the element symbols
Step 2 – write down the valency of each element
Step 3 – cross over the valency of each element
Step 4 – Simplify
Step 5 – Write down the formula.
4. Now write out the chemical formulae for the following 2 element compounds
a. Boron Fluoride
i. Aluminium oxide
b. Carbon sulphide
j. Magnesium phosphorus
c. Calcium iodide
k. Hydrogen sulphide
d. Hydrogen fluoride
l. Calcium nitride
e. Sodium sulphide
m. Carbon chloride
f. Lithium nitride
n. Barium oxide
g. Tin bromide
o. Potassium oxide
h. Strontium bromide
p. Calcium hydroxide.
Rule 4 - Simple Formulae for compounds containing group ions
Page 4 of your data booklet gives a list of group ions that contain more than one kind of
atom such as the ammonium ion, NH4+. This ion consists of a group of 5 atoms, one nitrogen
and 4 hydrogen atoms, with the whole group having a single positive charge. The valency of
a group ion is the same as the number of charges it has. For example, the ammonium ion
NH4+, with 1 positive charge, has a valency of one.
Example 1
Work out the simple chemical formula for ammonium chloride
Step 1
symbols
NH4
Cl
Step 2
ion charges
1
1
Step 3
cross over charges
(NH4)1
Cl1
Step 4
cancel out any common factor
(NH4)1
Cl1
Step 5
omit ‘1’ if present
NH4Cl
Example 2
Work out the simple chemical formula for ammonium carbonate
Step 1
symbols
NH4
CO3
Step 2
ion charges
1
2
Step 3
cross over charges
(NH4)2
(CO3)1
Step 4
cancel out any common factor
(NH4)2
CO3
Step 5
omit ‘1’ if present
(NH4)2CO3
5. Write simple chemical formulae for the following
a) Lithium hydroxide
j) Beryllium chromate
b) Potassium carbonate
k) Sodium dichromate
c) Calcium sulphate
l) Ammonium bromide
d) Strontium carbonate
m) Lithium hydrogensulphite
e) Potassium chromate
n) Potassium sulphate
f) Sodium sulphite
o) Aluminium phosphate
g) Lithium hydrogencarbonate
p) Ammonium nitrite
h) Sodium nitrate
q) Magnesium sulphite
i) Potassium phosphate
r) Sodium carbonate.
Rule 5 - Simple formulae for compounds using Roman Numerals
I
=
1
V
=
5
II
=
2
VI
=
6
III
=
3
VII
=
7
IV
=
4
Some elements such as the transition metals, can have more than one valency, for example
iron can have a valency of 2 or 3. Chemists use Roman numerals to show which valency is
being used.
Example 1
Work out the simple chemical formula for iron (II) oxide.
Step 1
symbols
Fe
O
Step 2
valency
2
2
Step 3
cross over valencies
Fe2
O2
Step 4
cancel out any common factor
Fe1
O1
Step 5
omit ‘1’ if present
FeO.
Example 2
Work out the simple chemical formula for iron (III) oxide.
Step 1
symbols
Fe
O
Step 2
valency
3
2
Step 3
cross over valencies
Fe2
O3
Step 4
cancel out any common factor
Fe2
O3
Step 5
omit ‘1’ if present
Fe2O3.
6. Write simple chemical formulae for the following
a) Iron (II) chloride
h) Silver (I) iodide
b) Nickel (II) sulphide
i) Bismuth (III) sulphide
c) Manganese (IV) oxide
j) Silver (II) fluoride
d) Tin (IV) chloride
k) Copper (I) oxide
e) Lead (II) iodide
l) Uranium (VI) fluoride
f) Lead (II) oxide
m) Manganese (VII) oxide
g) Antimony (V) oxide
n) Bismuth (III) hydride
Rule 6 - Simple formulae for compounds using brackets
If more than one of the same group ion is present in a chemical formula, then brackets are
placed around that ion, followed by a number to indicate how many of those ions are
present. For example the formula Ca(OH)2, indicates that, in calcium hydroxide, there are
2 hydroxide ions, OH-, for every calcium ion, Ca2+
Example
Work out the simple chemical formula for calcium hydroxide.
Step 1
symbols
Ca
OH
Step 2
ion charge
2
1
Step 3
cross over charge
Ca1
(OH)2
Step 4
cancel out any common factor
Ca1
(OH)2
Step 5
omit ‘1’ if present
Ca(OH)2.
7. Write simple chemical formulae for the following
a) Magnesium hydroxide
h) Ammonium sulphite
b) Barium phosphate
i) Barium hydrogensulphate
c) Beryllium nitrate
j) Aluminium hydroxide
d) Calcium hydrogencarbonate
k) Aluminium sulphate
e) Ammonium phosphate
l) Strontium nitrite
f) Magnesium nitrate
m) Calcium hydrogensulphite
g) Calcium nitrite
n) Ammonium carbonate.
Word Equations
In all chemical reactions one or more new substances are formed, the initial substances in
the reaction are called the reactants. The substances formed in the reaction are called the
products. Word equations can be written for any chemical reaction that shows the names
of the reactants and the products. In all chemical reactions, including word equations ‘+’
means ‘and’ and ‘
‘ means ‘reacts to produce’.
Example - methane is burned in oxygen to produce carbon dioxide and water.
Methane
CH4
8.
+
oxygen
carbon dioxide +
O2
CO2
water
H 2O
Write word equations for the following reactions and then underneath write the
chemical formula for each compound.
a) Sodium reacting with chlorine to give sodium chloride
b) Hydrogen reacting with bromine to give hydrogen bromide
c) Magnesium reacting with oxygen to give magnesium oxide
d) Calcium carbonate decomposing to give calcium oxide and carbon dioxide
e) Potassium hydroxide reacting with carbon dioxide to give potassium carbonate and
water.
f) Lithium oxide reacting with water to give lithium hydroxide.
g) Barium nitrate and sodium sulphate reacting to produce barium sulphate and sodium
nitrate
h) Zinc and copper (II) chloride reacting to produce copper and zinc (II) chloride.
Balancing Chemical Equations
During a chemical reaction atoms can be neither created nor destroyed. All of the atoms
present in the reactants are still present in the products. This means that to write out a
chemical equation properly we have to have the same number of each type of atom on each
side of the equation. This is called balancing equations
Example
In the complete combustion of methane, methane and oxygen react to produce carbon
dioxide and water. This can be written as shown
CH4
Count the number
Left:
H atoms
C atoms
O atoms
+
O2
CO2
+
H 2O
of atoms on each side of the equation
Right:
4
H atoms
1
C atoms
2
O atoms
2
1
3
Now you need to work out how to make each side balance. If we look at the hydrogen atoms
first, the only product produced from hydrogen is water.
The 4 hydrogen atoms in the methane molecule will produce 2 water molecules
CH4
+
O2
CO2
+
2H2O
This means that there will be 1 more oxygen on the right hand side from the extra
molecule of water
Left:
Right:
H atoms
4
H atoms
4
C atoms
1
C atoms
1
O atoms
2
O atoms
4
We must have used 2 molecules of oxygen to burn 1 molecule of methane
CH4
Left:
H atoms
C atoms
O atoms
+
4
1
4
2O2
CO2
Right:
+
2H2O
H atoms
C atoms
O atoms
4
1
4
Example 2
Balance the equation
H2
+
O2
H 2O
Count the number of atoms on each side of the equation
Left:
Right:
H atoms
2
H atoms
O atoms
2
O atoms
2
1
Now you need to work out how to make each side balance. If we look at the oxygen atoms
first,
The 2 oxygen atoms in the oxygen molecule will produce 2 water molecules
H2
+
O2
2H2O
This means that there will be 2 more hydrogen on the right hand side
Left:
Right:
H atoms
2
H atoms
4
O atoms
2
O atoms
2
We must have used 2 molecules of hydrogen with 1 molecule of oxygen to produce 2
molecules of water.
2H2
Left:
H atoms
O atoms
+
4
2
O2
2H2O
Right:
H atoms
O atoms
4
2
9. Balance the following chemical equations. Note some may naturally be balanced.
a. Mg + O2
MgO
b. K + I2
KI
c. H2+Cl2
HCl
d. PbO2 + H2
Pb +H2O
e. Mg + H2SO4
MgSO4 + H2
f. Li + H2O
LiOH +H2
g. KOH + CO2
K2CO3 + H2O
h. Cl2 + NaBr
NaCl + Br2
i. AgNO3
Ag + NO2 +O2
j. C2H4 + O2
CO2 + H2O
k. C3H6 + O2
CO2 + H2O
l. K2CO3 + HCl
KCl +H2O +CO2
m. Zn + HNO3
Zn(NO3)2 + H2
Formula Mass
The formula mass of a substance (or relative formula mass) is obtained by adding the
relative atomic masses of the elements in the formula.
Example 1
Calculate the formula mass of calcium chloride
Formula:
Formula mass:
CaCl2 (worked out using valencies)
40 + (35.5*2) = 111 (formula mass can be found on p4 of data book)
Example 2
Calculate the formula mass of ammonium sulphate
Formula:
Formula mass:
(NH4)2SO4 (worked out using ion group valencies)
([14+(1*4)]*2) + 32 + (16*4)
Work out the formula and then calculate the formula mass for the following:
a) Nitrogen,
b) Methane,
c) Potassium hydroxide,
d) Magnesium hydroxide,
e) Copper (II) sulphate
f) Ammonium carbonate
g) Lithium phosphate
The mole
The relative formula mass of a substance has no units, but in order to weigh out chemicals
we need to know the units. Chemists use a special term called the mole which simply gives
the units of grams to formula mass. In other words 1 mole of a substance is the same as
the formula mass expressed in grams.
Using the examples above 1 mole of calcium chloride is 111g and 1 mole of ammonium
sulphate is 132g.
For any substance the mass can be calculated from the number of moles and the formula
mass.
Mass of substance = number of moles x mass of one mole
= number of moles x formula mass in grams
Example
Calculate the mass of 2.5 moles of calcium carbonate
Mass in grams = number of moles X formula mass
Note the number of moles (from the question) = 2.5
Formula mass for CaCO3 need to work out
= 2.5 x [40+12+(16x3)] = 100g
Put this information into the above equation
Mass of CaCO3 in grams = 2.5x100
= 250g
We can rearrange this equation to work out the number of moles or the formula mass of
the substance if you have the other information
If you know the mass of a substance you can work out the formula mass from the formula
and data book and rearrange the equation to find the number of moles
Number of moles = mass of substance in grams
Formula mass
Example Calculate the number of moles of water in 100g of water.
Mass of water = 100g
Formula mass of water = [(2XH) + O)]
= [(2x1) +16)]
= 18 g
Number of moles = 100/18
= 5.56
Similarly if you know the mass of a substance in grams and the number of moles of this
substance you can rearrange the equation to work out the formula mass of a substance.
Formula mass = mass of substance in grams
Number of moles of substance
Example
Three moles of gas X have a mass of 132g. Calculate the formula mass of gas X and suggest
what it may be
Mass of water = 132g
Number of moles = 3
Formula mass = 132/3 = 44 the gas could possibly be CO2
Triangle of knowledge
The relationship between the mass of a substance (g), the number of moles of a substance
(n) and its formula mass (FM), can be summed up as follows
Mass in grams
m
Formula mass
n
FM
Number of moles
1. Calculate the mass of each of the following:
a. 1 mole of Argon,
b. 1 mole of magnesium nitride,
c. 1 mole of potassium nitrate,
d. 6 moles of propane
e. 3.5 moles of sodium sulphate
f. 0.25 moles of ammonium nitrate
g. 0.016 moles of calcium iodate
2. Calculate the number of moles in each of the following
a. 15g of hydrogen
b. 90g of ethane
c. 850g of silver (I) nitrate
d. 250g of ammonium chloride
e. 2.25 kg of ammonium sulphate
f. 24 kg of calcium nitrate
g. 1.80g of glucose.
3. Calculate the formula mass of each of the following
a. 2 moles of a substance that has a mass of 36g
b. 6 moles of a substance that has a mass of 180g
c. 4.4 moles of a substance that has a mass of 466.4g
d. 12.5 moles of a substance that has a mass of 925g
e. 8.7 moles of a substance that has a mass of 2.697Kg