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Names and formulae of compounds
Naming compounds
Most of the substances around you are compounds which have been made
by combining some of the 100 or so elements mentioned earlier.
Compounds are two or more different elements chemically bonded
together. The name of a chemical compound comes from simply combining
the names of the elements you reacted together to make your compound.
E.g.
sodium + chlorine  sodium chloride
iron + oxygen  iron oxide
The -ide is added to the end to tell us that 2 elements have joined
together, forming a compound. The metal always appears first in the name
(if there is one). If the compound in made of two non-metals joined
together, the element with the lower group number comes first. For
naming compounds, hydrogen is considered to be part of group 1.
E.g.
hydrogen + chlorine  hydrogen chloride
carbon + oxygen  carbon dioxide
the prefixes mon- and di- are used to identify different oxides where
more than 1 exists.
When a compound contains oxygen and 2 other elements, the compound
was not made by reacting all three elements together directly but by
another route. The name of compounds of this type (i.e with 2 elements
and also oxygen) contains the name of the first two elements (metal
first) and end in -ate to indicate the presence of oxygen.
E.g.
A compound containing copper, sulphur and oxygen is called
copper sulphate.
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masses and chemical equations
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Formulae
Writing chemical formulae correctly is not as difficult as people think.
The chemical formula is made up using the symbol on the periodic table.
Make sure you get the correct symbol. For simple molecules the formula
shows the actual number of each type of atom present. However for ionic
substances and macromolecules, which have giant structures, the formula
shows the ratio of ions or atoms of each type. We can use valencies to
help us work out the formula of a compound.
The valency of an element is the number of electrons an atom
has to lose, gain or share to form a compound and is therefore
depends on the group it is in on the periodic table. Some valencies are
shown in the table below.
Element
Group 1
Group 2
Group 3
Group 4
Group 5
Group 6
Group 7
To form a compound
Lose 1 eLose 2 eLose 3 eShare 4 eShare or gain 3 eShare or gain 2 eShare or gain 1 e-
Valency
1
2
3
4
3
2
1
To work out the formula using valencies e.g. sodium oxide
1
Write down the valencies of the two elements.
Sodium = 1
2
Write down the symbols of the elements in the order they appear
in the name.
Na
3
oxygen = 2
O
Add subscript numbers after each element so that the valencies
balance.
We need two Na’s since O is already 2
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masses and chemical equations
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Answer
Na2O
Another way to work out formulae is by balancing charges on ions since
the final compound is electrically neutral. E.g Aluminium oxide
1
Write down the charges on the two ions.
Aluminium = 3+
2
oxygen = 2-
Imagine them on a ‘charge balancing see-saw’ and add ions to both
sides until the total on the positive side is equal to the total on the
negative side.
Al3+ Al3+
O2- O2- O2-
Hence the formula contains 3 x Al and 2 x O
Answer Al2O3
Some compounds have common names which are not helpful in
understanding which elements they are made from and you simply have to
learn these. E.g. Water H2O, methane CH4 and ammonia NH3.
Once you have worked out chemical formulae you will most likely want to
use them to write balanced equations.
Balancing is done by placing numbers called coefficients in front of the
formulas for the compounds/elements. For example, ‘O2‘ means there is
one oxygen molecule involved in a reaction but ‘2O2’ would mean there are
two.
Example:
CH4(g) + O2(g)
CO2(g) + H2O(g)
Starting with the first element (C) we can see that there is 1C on the left
and also 1 on the right so they are balanced. The equation is still
unbalanced as there are 4 ‘H’ on the left but only 2 ‘H’ on the right. This
must be corrected by placing a ‘2’ in front of the ‘H2O’ so there are now 2
waters:
CH4(g) + O2(g)
CO2(g) + 2H2O(g)
Note: You can’t change the little numbers (ie the 2 in H2O ) as this
would no longer be water.
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masses and chemical equations
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Now the ‘H’ balances but there 4 ‘O’ on the right and only 2 on the left.
This must be balanced by placing a ‘2’ in front of the ‘O2’ so that there
are 2 oxygen molecules:
CH4(g) + 2O2(g)
CO2(g) + 2H2O(g)
Now there is 1 ‘C’, 4 ‘H’ and 4 ‘O’ on each side so it balances.
In ionic equations, we tend to look only at the ions that actually change.
For example, when iron reacts with copper sulphate to form iron sulphate
and copper the equation is:
Fe(s) + Cu2+(aq) + SO42-(aq)
Fe2+(aq) + SO42-(aq) + Cu(s)
Remember that when a salt is in solution the lattice breaks up and its ions
separate. In this case, the sulphate ion (SO42-) remains unchanged (we call
it a spectator ion) so it can be left out of the equation to give:
Fe(s) + Cu2+(aq)
Fe2+(aq) + Cu(s)
THE MASSES OF ATOMS AND MOLECULES
MASS NUMBER AND RELATIVE ATOMIC MASS
The mass number of an isotope is equal to the number of protons plus
neutrons in one atom and is, therefore, a whole number.
In round numbers, mass number and relative atomic mass are equal, but
the relative atomic mass is not an exact whole number, partly because
protons and neutrons do not have exactly the same mass and partly
because of the existence of isotopes.
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masses and chemical equations
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RELATIVE ATOMIC MASS
Because of the incredibly small size of atoms, it is not possible to weigh
them directly. Instead the masses of atoms are related to an arbitrary
standard; for this purpose the mass of one atom of 12C is taken to be
12.0000. The mass of any other atom compared to that of the carbon
atom is called it’s RELATIVE ATOMIC MASS and is given the symbol Ar.
The relative atomic mass of an element is defined as the average mass of one
atom of the element compared with one twelfth the mass of an atom of 12C.
N.B. Relative atomic mass is a ratio, so it is simply a number and has no
units.
The 12C atom contains 6 protons, 6 neutrons and 6 electrons. The
hydrogen atom contains 1 proton and 1 electron. Since the mass of an
electron is negligible compared to that of a proton or a neutron, the
hydrogen atom has only 1/12 the mass of a carbon atom; therefore the
relative atomic mass of hydrogen is 1.
Similarly, the relative mass of an oxygen atom, which contains 8 protons,
8 neutrons and 8 electrons, is 16.
RELATIVE FORMULA MASS
We can use the Ar to calculate the relative formula mass (given the
symbol Mr) of compounds regardless of whether they exist as individual
molecules or as a giant ionic lattice. For CO2 the molecule contains a
carbon atom and two oxygen atoms and so 1 molecule of CO2 has a relative
formula mass of 12+16+16=44. Similarly the formula of NaCl shows 1
sodium atom for every chlorine atom and so the Mr of NaCl is
23+35.5=58.5.
The method for calculating Mr is straightforward even if the formula of
a substance in not.
Ammonium sulphate has the formula (NH4)2SO4
Mr = (2x14)+(8x1)+32+(4x16) = 132
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masses and chemical equations
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Calculating quantities
During chemical reactions elements always react in the same ratio to
form compounds. Mass is also conserved so the total mass of reactants =
total mass of products.
Consider
copper + oxygen
copper oxide
If 64g of copper react with 16g of oxygen to form copper oxide
What mass of oxygen will 32g of copper react with?
Ans …………………
What mass of copper will 64g of oxygen react with?
Ans …………………
What mass of copper oxide forms when 64g of copper react with 16g of
oxygen?
Ans ………………
What mass of copper and what mass of oxygen must react to produce
8.0g of copper oxide?
Ans ………………
Consider
calcium carbonate
calcium oxide + carbon dioxide
If 100g of calcium carbonate react to produce 56g of calcium oxide
and 44g of carbon dioxide
What mass of calcium oxide will 60g of calcium carbonate produce?
Ans …………………
What mass of carbon dioxide will 20g of calcium carbonate produce?
Ans …………………
What mass of calcium carbonate will be needed to form 5.6g of calcium
oxide and 4.4g of carbon dioxide
Ans ………………
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THE PERCENTAGE COMPOSITION OF COMPOUNDS
A pure chemical compound has a fixed chemical composition and is
represented by a fixed formula. This formula conveys information about:
a) the elements present in the substance
b) the mass of 1 mole of the substance (Mr expressed in g)
c) the number of moles of each element present in 1 mole of the
substance
The percentage composition of a compound is the % contribution by mass
which each component element makes to the substance.
CALCULATION OF % COMPOSITION
Example: Calculate the % composition by mass of ammonium nitrate.
1. Work out the formula of the compound. The formula of ammonium nitrate is
NH4NO3.
2. Calculate the relative molecular/formula mass of the compound.
Mr (NH4NO3) = 14 + (4x1) + 14 + (3x16) = 80
 80g NH4NO3 contains
28g nitrogen
4g hydrogen
48g oxygen
3. Express the mass of each element as a percentage of the molar mass.
%N = 28 x 100 = 35.0%
80
%H =
4 x 100 = 5.0%
80
%O = 48 x 100 = 60.0%
80
Answers are usually given to 1 d.p.
4. Check that the percentages add up to 100%.
N.B. Errors from rounding up or down may give answers between 99.8 and 100.2%,
How
pure are substances?
which
is satisfactory.
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PURITY
We make use of raw materials such as limestone, crude oil and metal ores
which are obtained from the earth’s crust. These materials contain other
substances and are therefore not pure. Purity is normally stated as a
percentage.
Percentage purity = mass of pure substance present
total mass (including impurities)
x
100
Example
20 tonnes of impure copper is purified by electrolysis. 18 tonnes of pure
copper is obtained. What is the % purity of the copper before it was
refined.
Percentage purity =
18
20
x
100
=
90%
An example for you to do
Limestone is an impure form of calcium carbonate. If 315g of limestone
contain 300g of calcium carbonate, calculate the % purity of the
limestone.
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masses and chemical equations
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Topic 5 Reacting masses and equation
Summary questions
1
2
3
Write the correct chemical formulae for the following compounds.
Sodium chloride …………………………………………………
Sodium oxide …………………………………………………
Magnesium fluoride …………………………………………………
Potassium sulphate …………………………………………………
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masses and chemical equations
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4
5
Write down the ratio of atoms present in each of the following compounds.
Copper oxide CuO …………………………………………………………..
Potassium oxide K2O …………………………………………………………
Calcium hydroxide Ca(OH)2 ………………………………………………….
Ammonium sulfate (NH4)2SO4 ………………………………………………..
6
Balance the following equations
1
Li
+
S

Li2S
2
K
+
P

K3P
3
H2
+
Br2

HBr
4
Cu
+
O2

CuO
C2H6
+
O2

CO2
5
iGCSE TOPIC 10.5: Reacting
masses and chemical equations
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+
H2O
7
Consider the following reaction
C
+
O2

CO2
If 12g of carbon burns completely to produce 44g of carbon dioxide, how much
carbon dioxide can be made by burning 1.2g of carbon?
………………………………………………………………………………………
………………………………………………………………………………………
………………………………………………………………………………………
8
Consider the following reaction
S
+
O2

SO2
How much sulfur dioxide can be made when 8g of sulfur burns completely in
oxygen? Show your working.
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
9
Consider the following reaction
4K
+
O2

2K2O
How much potassium oxide can be made when 39g of potassium burns completely in
oxygen? Show your working.
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
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10 Calculate the % by mass of hydrogen, sulphur and oxygen in sulphuric acid
(H2SO4). Show your working
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
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masses and chemical equations
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