NAT 5 Chemistry
Calculations
Chemical Formula
Working out the correct chemical formulae is important because it is the first stage in some
calculation
Chemical Formulae from Prefix
Some help in writing the correct chemical formulae is often given by the following prefixes:
mono
di
tri
tetra
- one
- two
- three
- four
Examples:
Carbon monoxide
CO
Sulphur dioxide
Phosphorus trichloride
Dinitrogen tetraoxide
SO2
PCl3
N2O4
Write chemical formulae for the following five compounds:
1. Carbon dioxide
2. Nitrogen dioxide
3. Carbon disulphide
4. Carbon tetrachloride
5. Nitrogen monoxide
The cross-over method
The valency of an element tells us how many atoms of another element in a compound that
particular atom will combine with.
Remember, hydrogen has a valency of one
Transition Metal Elements
Transition metal elements can have more than one valency. The valency to be used for any
transition metal element is written in Roman Numerals, in brackets, after the name of the
transition metal element.
Roman Numerals:
(i) =
(ii) =
(iii) =
(iv) =
1
2
3
4
Valency of transition metal elements
The cross-over method for –ide compounds
Example 1:
Calcium chloride
Elements
Valency
calcium
2
chlorine
8-7 = 1
Write the symbol with the valency number at the top right hand corner of the symbol.
Cross-over
Reduce ratio
Chemical formula
Ca2
1
1
Cl1
2
2
CaCl2
Example 2:
Aluminium oxide
Symbols/valency
Cross-over
Reduce ratio
Al3
2
2
Chemical formula
O2
3
3
Al2O3
Write chemical formulae for the following ten –ide compounds:
6.
7.
8.
9.
10.
Sodium chloride
Magnesium sulphide
Aluminium fluoride
Calcium nitride
Zinc(ii) bromide
11.
12.
13.
14.
15.
Nickel(ii) chloride
Potassium sulphide
Lithium nitride
Iron(iii) oxide
Magnesium nitride
The cross-over method for –ite and -ate compounds
Example 1:
Copper(ii) Sulphate
The sulphate ion is a complex ion.
Check the correct formula and ion charge of any complex ion in the table, ‘formulae of
selected ions…’ on page 4 of the SQA data booklet.
SO42-
Sulphate ion is
Place the formula of the ion in brackets (SO4) with the valency of the ion represented by the
charge on the ion i.e. 2.
Elements/complex ions
copper(ii)
Valency
Sulphate ion
2
2
Write the symbol with the valency number at the top right hand corner of the symbol.
Symbol/valency
Cross-over
Reduce ratio
Chemical formula
Example 2:
Symbols/valency
Cross-over
Reduce ratio
Chemical formula
Cu2
2
1
(SO4)2
2
1
Cu(SO4) or CuSO4
Ammonium Phosphate
(NH4)1
3
3
(PO4)3
1
1
(NH4)3 PO4
Write chemical formulae for the following ten –ite/ate compounds:
16.
17.
18.
19.
20.
Ammonium nitrate
Magnesium phosphate
Iron(iii) sulphate
Calcium nitrate
Boron carbonate
21.
22.
23.
24.
25.
Calcium hydroxide
Potassium sulphate
Lithium carbonate
Calcium sulphite
Magnesium nitrate
Ionic Formulae
Ionic Formulae are the chemical formulae of ionic compound including the charges on the
ions in the compound.
Example 1:
Calcium chloride
Elements
calcium
Ion
Ca2+
Valency
2
chlorine
Cl8-7 = 1
Write the symbol of the ion with the valency number at the top right hand corner of the
brackets surrounding the ion.
(Ca2+)2
Cross-over
Reduce ratio
(Cl-)1
1
1
2
2
Ca2+(Cl-)2
Chemical formula
Example 2:
Ion/valency
Ammonium Phosphate
(NH4+)1
(PO43-)3
-over
3
1
Reduce ratio
3
1
Chemical formula
(NH4+)3 PO43-
Write ionic formulae for the following ten –ite/ate compounds:
26.
27.
28.
29.
30.
Ammonium nitrate
Magnesium phosphate
Iron(iii) sulphate
Calcium nitrate
Boron carbonate
31.
32.
33.
34.
35.
Calcium hydroxide
Potassium sulphate
Lithium carbonate
Calcium sulphite
Magnesium nitrate
Chemical Equations
Balanced equations indicate the mass of reactants and products involved in a particular
reaction.
Word Equations.
Chemical reactions can be represented by chemical equations. These equations inform
scientists of what is happening during a chemical reaction.
Example:
Sodium reacts with oxygen to produce sodium oxide.
sodium + oxygen → sodium oxide
Write word equations for the following reactions:
36 When magnesium (a grey metal) burns in air it gives out a bright light and lots
of heat. A white powder called magnesium oxide is also produced.
37 When the colourless gas hydrogen is and the green gas chlorine are mixed and
a bright light is shone on the mixture, an explosion occurs and white fumes of
hydrogen chloride are formed.
38 When chalk (calcium carbonate) is added to sulphuric acid, calcium sulphate
and water are produced. Carbon dioxide is also produced.
39 When grey magnesium metal is added to a blue solution of copper(ii) sulphate,
brown copper solid forms on the magnesium and the blue solution fades to a
clear solution of magnesium sulphate.
40 When hydrochloric acid is added to sodium hydroxide no change was seen to
have taken place. The products of this reaction are sodium chloride and water.
Formula Equations.
Formula equations are a short hand way of showing chemicals reacting and being produced
using their chemical formulae.
Example:
Copper reacts with silver(i) nitrate to form silver and copper(ii) nitrate.
Word Equation
Copper + silver(i) nitrate → silver + copper(ii) nitrate
Formula equation
Cu + AgNO3 → Ag + Cu(NO3)2
Write formula equations for each of the following reactions:
41 magnesium + sulphur dioxide → magnesium oxide + sulphur
42 calcium + water → calcium hydroxide + hydrogen
43 calcium hydroxide + sulphuric acid → calcium sulphate + water
44 potassium carbonate + hydrochloric acid →
potassium chloride + water + carbon dioxide
45 potassium sulphate solution + barium nitrate solution →
potassium nitrate solution + barium sulphate solution
Balanced Equations.
A formula equation is balanced if the number and type of atoms on the left hand side of the
equation exactly equal the number and type of atoms on the right hand side of the equation.
Example:
When hydrogen reacts with chlorine, hydrogen chloride is formed.
Word equation
hydrogen + chlorine → hydrogen chloride
Formula equation
H2 + Cl2 → HCl
Balanced equation
H2 + Cl2 → 2HCl
Balance the following chemical equations:
46. Mg(s) + O2(g) → MgO(s)
47. NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + H2O(l)
48. AgNO3(aq) + BaCl2(aq) → Ba(NO3)2(aq) + AgCl(s)
49. Na(s) + H2O(l) → NaOH(aq) + H2(g)
50. Al(s) + Cl2(g) → AlCl3(s)