O Level Cambridge Chemistry 3 hours 23 questions

Theory Questions

3.1 Formulae
Formulae / Empirical Formulae & Formulae of Ionic Compounds / Writing
Equations

Easy (6 questions) /39 Scan here to return to the course

or visit savemyexams.com
Medium (11 questions) /83

Hard (6 questions) /52

Total Marks /174

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 1
Easy Questions
1 (a) When cobalt(II) oxide, CoO, is heated in air an oxide with the formula Co3O4 is formed.

Balance the equation for this reaction.

…....... CoO + O2 → 2Co3O4

(1 mark)

(b) Sulfuric acid is a compound.

The formula of sulfuric acid is H2SO4.

Complete Table 1.1 to calculate the relative molecular mass of sulfuric acid.

Table 1.1

relative atomic
atom number of atoms
mass
hydrogen 2 1 2×1=2
sulfur
oxygen

relative molecular mass = .............................

(2 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 2
(c) Hydrogen is a fuel which can be obtained from water by electrolysis.

Refinery gas and petrol are fuels obtained by the fractional distillation of petroleum.

Complete the equation for the burning of hydrogen.

.......... H2 (g) + O2 (g) → ..........H2O (l)

(1 mark)

(d) State the meaning of (g) and (l).

(g):

(l):

(2 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 3
2 (a) In a blast furnace used for the extraction of iron, carbon reacts with oxygen from the air
to form carbon monoxide.

Complete the chemical equation for this reaction.

....C + ..... → 2CO

(2 marks)

(b) The structure of a compound of iron is shown.

Deduce the molecular formula of this compound to show the number of iron, carbon
and oxygen atoms.

(1 mark)

(c) Iodine reacts with aqueous sodium thiosulfate, Na2S2O3.

Balance the chemical equation for this reaction.

.....Na2S2O3 + I2 → Na2S4O6 + .....NaI

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 4
 (2 marks)

(d) Complete the table to calculate the relative formula mass of anhydrous copper(II) sulfate,
CuSO4.

Use your Periodic Table to help you.

relative atomic
type of atom number of atoms
mass
copper 1 64 1 × 64 = 64
sulfur
oxygen

relative formula mass =_______________

(2 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 5
3 (a) This question is about iron and compounds of iron.

Iron can be converted into steel in a basic oxygen converter.

Oxygen is blown into the impure molten iron to remove some of the impurities.

Oxygen reacts with the carbon in the impure iron to form carbon dioxide.

Write a chemical equation for this reaction.

(1 mark)

(b) This question is about bromine and compounds of bromine.

Bromine reacts with hydrogen sulfide, H2S.

Complete the chemical equation for this reaction.

....... + H2S → .....HBr + S

(2 marks)

(c) This question is about cobalt and compounds of cobalt.

Complete the table to calculate the relative formula mass of anhydrous cobalt(II) sulfate,
CoSO4.
Use your Periodic Table to help you.

relative atomic
type of atom number of atoms
mass
cobalt
sulfur 1 32 1 × 32 = 32
oxygen

relative formula mass = ______________

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 6
 (2 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 7
4 (a) The names and formulae for some compounds are shown.

aluminium nitrate, Al(NO3)3

magnesium nitrate, Mg(NO3)2
sodium nitrate, NaNO3

Deduce the formula for calcium nitrate.

(1 mark)

(b) Sodium reacts with molten sodium hydroxide.

Complete the chemical equation for this reaction.

2Na + ......NaOH → ......Na2O + H2

(2 marks)

(c) The structure of a carboxylic acid is shown.

Deduce the formula of this carboxylic acid to show the number of atoms of carbon,
hydrogen and oxygen.

(1 mark)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 8
5 (a) This question is about reactions involving calcium compounds.

Heating calcium carbonate, CaCO3, produces calcium oxide, CaO, and one other gaseous
product.

Write the word equation for this reaction.

(2 marks)

(b) Write a balanced chemical equation for heating calcium carbonate, CaCO3, to form
calcium oxide, CaO, and one other gaseous product.

(1 mark)

(c) Calcium hydroxide, Ca(OH)2 can be made by the reaction of calcium oxide, CaO, with
water.

CaO + H2O → Ca(OH)2

112 g of calcium oxide reacts with 36 g of water.

Calculate the mass of calcium hydroxide that is produced.

____________________ g

(1 mark)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 9
(d) Calcium hydroxide can be used to test for the presence of carbon dioxide.

If carbon dioxide is present, the calcium hydroxide will form a milky white precipitate of
calcium carbonate.

The formula of calcium carbonate is CaCO3

Calculate the relative formula mass (Mr) of calcium carbonate.

Relative atomic masses (Ar): C = 12 O = 16 Ca = 40

(2 marks)

(e) Calculate the percentage of oxygen in calcium carbonate, CaCO3, using the following
equation.

Ar × number of atoms
Percentage by mass = × 100
Mr of the compound

(2 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 10
6 (a) Extended Only
Compound X is a colourless liquid at room temperature.

A sample of pure X was slowly heated from –5.0 °C, which is below its melting point, to
90 °C, which is above its boiling point. Its temperature is measured every minute and the
results are represented on the graph.

i) Complete the equation for the equilibrium present in the region BC.

X(s) ⇌ ....................
[1]

ii) What is the significance of temperature t°C?

[1]

iii) What is the physical state of compound X in the region EF?

[1]

iv) What would be the difference in the region BC if an impure sample of X had been
used?
[1]

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 11
 (4 marks)

(b) Compound X is a hydrocarbon. It contains 85.7% of carbon. The mass of one mole of X is
84g.

i) What is the percentage of hydrogen in the compound?

[1]

ii) Calculate the empirical formula of X. Show your working.

[3]

iii) What is the molecular formula of compound X?

[1]

(5 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 12
Medium Questions
1 (a) Use your copy of the Periodic Table to help you answer these questions.

Predict the formula of each of the following compounds.

i) aluminium fluoride
[1]

ii) arsenic oxide

[1]

iii) silicon bromide

[1]

(3 marks)

(b) Deduce the formula of each of the following ions.

i) phosphide
[1]

ii) barium
[1]

iii) francium
[1]

(3 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 13
(c) Draw a diagram showing the arrangement of the valency electrons in one molecule of
the covalent compound carbon dioxide.

Use o to represent an electron from a carbon atom.

Use x to represent an electron from an oxygen atom.

(3 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 14
2 (a) The law of constant composition states that all pure samples of a compound contain the
same elements in the same proportion by weight.

A typical experiment to test this law is to prepare the same compound by different
methods and then show that the samples have the same composition.

Methods of making copper(II) oxide include:

heating copper carbonate,

heating copper hydroxide,
heating copper nitrate,
heating copper foil in air.

Complete the following equations.

i) CuCO3 → ............. + .............

[1]

ii) Cu(OH)2 → ............. + .............

[1]

iii) 2Cu(NO3)2 → ............. + 4NO2 + .............

[2]

(4 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 15
(b) Copper oxide can be reduced to copper by heating in hydrogen.

i) What colour change would you observe during the reduction?

[1]

ii) Explain why the copper must be allowed to cool in hydrogen before it is exposed to
air.
[2]

iii) Name another gas which can reduce copper(II) oxide to copper.
[1]

iv) Name a solid which can reduce copper(II) oxide to copper.

[1]

(5 marks)

(c) The table below shows the results obtained by reducing the copper(II) oxide produced by
different methods to copper.

i) Complete the table.

source of mass of mass of percentage

copper(II) oxide copper(II) oxide / g copper / g copper / %
CuCO3 2.37 1.89 79.7
Cu(OH)2 2.51 1.99
Cu(NO3)2 2.11 1.68
Cu and O2 2.29 1.94

[2]

ii)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 16
One of the samples of copper(II) oxide is impure.

Identify this sample and suggest an explanation why the percentage of copper in
this sample is bigger than in the other three samples.
[2]

(4 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 17
3 (a) Iron pyrite, FeS2, is known as Fool’s Gold because it is a shiny yellow solid which is similar
in appearance to gold. Iron pyrite is an ionic compound. Gold is a metallic element.

Iron pyrite, FeS2, contains positive and negative ions. The positive ion is Fe2+.
Deduce the formula of the negative ion.

(1 mark)

(b) A student is provided with a sample of iron pyrite and a sample of gold.

Suggest how the student could distinguish between the two substances.

(2 marks)

(c) Sulfur dioxide is produced on a large scale by heating iron pyrite strongly in air. The iron
pyrite reacts with oxygen in the air producing iron(III) oxide, Fe2O3, and sulfur dioxide.

Construct a chemical equation for the reaction between iron pyrite and oxygen.

(2 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 18
4 (a) Element X can undergo the following physical changes.

Element X is a Group III metal. It burns in air to form an oxide X2O3.

Write a symbol equation for this reaction.

(2 marks)

(b) Aqueous silver nitrate is added to aqueous magnesium chloride.

A white precipitate forms.

Write an ionic equation for this reaction. Include state symbols.

(2 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 19
(c) Complete the chemical equation for the reaction of chlorine with phosphorus.

......Cl2 + 2P → ......PCl5

(2 marks)

(d) Aqueous potassium iodide reacts with aqueous copper(II) sulfate to produce iodine.

Balance the chemical equation for this reaction.

KI + CuSO4 → CuI + I2 + K2SO4

(2 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 20
5 (a) This question is about elements X, Y and Z.

An atom of element X is represented as 34

16 X

What is the electronic structure of the ion X2–?

(1 mark)

(b) This question is about elements X, Y and Z.

Identify the atom against which the relative masses of all other atoms are compared.

(1 mark)

(c)

Magnesium oxide, MgO, is formed when magnesium burns in oxygen.

Write the chemical equation for the reaction that occurs when magnesium burns in
oxygen.

(2 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 21
6 (a) The structures of five organic compounds, A, B, C, D and E, are shown.

Answer the questions that follow.

Each letter may be used once, more than once or not at all.

Give the letter of the compound that has the empirical formula CH2.

(1 mark)

(b) An endothermic reaction occurs when calcium nitrate is heated.

Balance the equation for this reaction.

.....Ca(NO3)2 → .....CaO + .....NO2 + .....O2

(1 mark)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 22
7 (a) This question is about iron and iron compounds.

Iron reacts with chlorine to form iron(III) chloride, Fe2Cl6.

Balance the chemical equation for this reaction.

.....Fe + .....Cl2 → Fe2Cl6

(2 marks)

(b) The structure of an ion is shown.

Deduce the molecular formula of this ion to show the number of iron, carbon and
nitrogen atoms.

(1 mark)

(c) This question is about chlorine and compounds of chlorine.

Chlorine reacts with warm turpentine, C10H16.

Balance the chemical equation for this reaction.

C10H16 + 8Cl2 → .....C + .....HCl

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 23
 (2 marks)

(d) The equation shows the effect of heat on anhydrous zinc sulfate.

heat
ZnSO4 ⎯⎯⎯⎯ ZnO + SO3

When 12.60 g of anhydrous zinc sulfate is heated, the mass of zinc oxide formed is 6.34
g.

Calculate the mass of zinc oxide formed when 63.0 g of anhydrous zinc sulfate is heated.

mass of zinc oxide = _______________

(1 mark)

(e) Complete the table to calculate the relative formula mass of anhydrous zinc sulfate,
ZnSO4. Use your Periodic Table to help you.

relative atomic
type of atom number of atoms
mass
zinc 1 65 1 × 65 = 65
sulfur
oxygen

relative formula mass = _________________

(2 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 24
8 (a) Quantities of chemicals, expressed in moles, can be used to find the formula of a
compound, to establish an equation and to determine reacting masses.

A compound contains 72% magnesium and 28% nitrogen. What is its empirical formula?

(2 marks)

(b) A compound, Al4C3, contains only aluminium and carbon. 0.03 moles of this compound
reacted with excess water to form 0.12 moles of Al(OH)3 and 0.09 moles of CH4.

Write a balanced equation for this reaction.

(1 mark)

(c) 0.07 moles of silicon reacts with 25 g of bromine.

Si + 2Br2 → SiBr4

i) Which one is the limiting reagent? Explain your choice.

[3]

ii) How many moles of SiBr4 are formed?

[1]

(4 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 25
9 (a) A compound, X, contains 55.85% carbon, 6.97% hydrogen and 37.18% oxygen.

i) How does this prove that compound X contains only carbon, hydrogen and
oxygen?
[1]

ii) Use the above percentages to calculate the empirical formula of compound X.
[2]

iii) The Mr of X is 86.

What is its molecular formula?

[2]

(5 marks)

(b) i) Bromine water changes from brown to colourless when added to X.

What does this tell you about the structure of X?

[1]
ii) Magnesium powder reacts with an aqueous solution of X. Hydrogen is evolved.

What does this tell you about the structure of X?

[1]
iii) X contains two different functional groups.

Draw a structural formula of X.

[1]

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 26
 (3 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 27
10 (a) Titanium is a transition element. It is isolated by the following reactions.

titanium ore → titanium(IV) oxide → titanium(IV) chloride → titanium

TiO2 TiCl4 Ti

Why is it usually necessary to include a number in the name of the compounds of

transition elements?

(1 mark)

(b) Titanium(IV) chloride is made by heating the oxide with coke and chlorine.

TiO2 + 2Cl2 ⇌ TiCl4 + O2

2C + O2 ⇌ 2CO

Explain why the presence of coke ensures the maximum yield of the metal chloride.

(2 marks)

(c) Explain why the change, titanium(IV) chloride to titanium, is reduction.

(1 mark)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 28
(d) The titanium ore contains 36.8% iron, 31.6% titanium and the remainder is oxygen.

i) Determine the percentage of oxygen in this titanium compound.

[1]

ii) Calculate the number of moles of atoms for each element.

The number of moles of Fe is shown as an example.

Number of moles of Fe = 36.8/56 = 0.66

[1]

iii) What is the simplest ratio for the moles of atoms, Fe : Ti : O?

[1]

iv) What is the formula of this titanium compound?

[1]

(4 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 29
11 (a) Across the world, food safety agencies are investigating the presence of minute traces of
the toxic hydrocarbon, benzene, in soft drinks. It is formed by the reduction of sodium
benzoate by vitamin C.

Sodium benzoate is a salt, it has the formula C6H5COONa. It can be made by the
neutralisation of benzoic acid by sodium hydroxide.

i) Deduce the formula of benzoic acid.

[1]

ii) Write a word equation for the reaction between benzoic acid and sodium
hydroxide.
[1]

iii) Name two other compounds that would react with benzoic acid to form sodium
benzoate.
[2]

(4 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 30
(b) Benzene contains 92.3% of carbon and its relative molecular mass is 78.

i) What is the percentage of hydrogen in benzene?

[1]

ii) Calculate the ratio of moles of C atoms: moles of H atoms in benzene.

[1]

iii) Calculate its empirical formula and then its molecular formula.

The empirical formula of benzene is ...................

The molecular formula of benzene is ...................
[2]

(4 marks)

(c) The structural formula of Vitamin C is drawn below.

i) What is its molecular formula?

[1]

ii) Name the two functional groups which are circled.

[2]

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 31
 (3 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 32
Hard Questions
1 (a) Fluorine forms both ionic and covalent compounds.

Magnesium reacts with fluorine to form the ionic compound magnesium fluoride.

The electronic structures of an atom of magnesium and an atom of fluorine are shown.

What is the formula of magnesium fluoride?

(1 mark)

(b) When copper is reacted with hot concentrated sulfuric acid, sulfur dioxide gas is formed.
Balance the chemical equation for this reaction.

Cu + .......H2SO4 → CuSO4 + SO2 + .......H2O

(2 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 33
(c) When sulfuric acid reacts with ammonia the salt produced is ammonium sulfate.

Write the chemical equation for this reaction.

(2 marks)

(d) Barium sulfate is an insoluble salt.

Barium sulfate can be made from aqueous ammonium sulfate using a precipitation
reaction.

Write an ionic equation for this precipitation reaction. Include state symbols.

(2 marks)

(e) Oxygen is produced by the decomposition of hydrogen peroxide. Manganese(IV) oxide is

the catalyst for this reaction.

Oxygen can also be produced by the decomposition of potassium chlorate(V), KClO3.

The only products of this decomposition are potassium chloride and oxygen.

Write a chemical equation for this decomposition.

(2 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 34
2 (a) This question is about titanium.

Titanium is a transition metal that is extracted from titanium dioxide in a two-stage

industrial process.

In the first stage, titanium dioxide reacts with carbon and chlorine to form titanium
tetrachloride and carbon monoxide.

Write the balanced symbol equation for this reaction.

(3 marks)

(b) Explain how your chemical symbol equation in part (a) demonstrates the law of
conservation of mass.

(1 mark)

(c) Identify two hazards associated with Stage 1.

(2 marks)

(d) Calculate, to three significant figures, the percentage by mass of chlorine in titanium
tetrachloride.

(Ar: Ti = 48 Cl = 35.5)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 35
 (2 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 36
3 (a) Magnesium displaces copper from copper sulfate solution.

Write the balanced symbol equation for the reaction.

You should include state symbols.

(2 marks)

(b) State two changes that would be observed during the displacement reaction.

(2 marks)

(c) Write the ionic equation for the displacement of copper from copper sulfate by
magnesium. You should include state symbols.

(2 marks)

(d) Displacement reactions are examples of redox reactions.

Explain why the displacement reaction between magnesium and copper sulfate is both
reduction and oxidation.

(2 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 37
4 (a) This question is about lead nitrate.

Lead nitrate is an ionic compound.

It consists of Pb2+ and NO3- ions.

What is the chemical formula of lead nitrate?

(1 mark)

(b) Calculate the percentage by mass of oxygen in lead nitrate, Pb(NO3)2.

(2 marks)

(c) A displacement reaction occurs between solutions of lead nitrate and potassium iodide,
KI, to form solid lead(II) iodide and aqueous potassium nitrate.

Write the balanced symbol equation, including state symbols, for this reaction.

(3 marks)

(d) Table 1 summarises the solubility of a selection of ionic compounds in water.

Table 1

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 38
 Soluble Insoluble
All nitrates
Most common chlorides Silver chloride, lead chloride
Most common sulfates Lead sulfate, barium sulfate, calcium sulfate
Sodium carbonate, potassium carbonate,
Most common carbonates
ammonium carbonate

The displacement reaction between lead nitrate and potassium iodide forms a yellow
precipitate.

Justify which chemical is responsible for the yellow colour.

(2 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 39
5 (a) The relative atomic mass, Ar, for each element is shown in the Periodic Table.

Define the term relative atomic mass.

(2 marks)

(b) The element gallium has a relative atomic mass of 69.735 and only contains two
isotopes.

A sample of gallium contained the isotope 69Ga, with a relative abundance of 63.25 %.

Calculate the mass number of the other isotope.

You must show all your working.

(2 marks)

(c) Define the term empirical formula.

(1 mark)

(d) An unknown compound contains carbon, hydrogen and oxygen.

It consists of 18% carbon,and 73 % oxygen.

Calculate the empirical formula of the unknown compound

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 40
 (3 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 41
6 (a) Until recently, arsenic poisoning, either deliberate or accidental, has been a frequent
cause of death. The symptoms of arsenic poisoning are identical with those of a common
illness, cholera. A reliable test was needed to prove the presence of arsenic in a body.

In 1840, Marsh devised a reliable test for arsenic

Hydrogen is formed in this reaction. Any arsenic compound reacts with this hydrogen to
form arsine which is arsenic hydride, AsH3.

i) The mixture of hydrogen and arsine is burnt at the jet and arsenic forms as a black
stain on the glass.
Write an equation for the reaction which forms hydrogen.
[2]
ii) Draw a diagram which shows the arrangement of the outer electrons in one
molecule of the covalent compound arsine.

The electron distribution of arsenic is 2 + 8 + 18 + 5.

Use x to represent an electron from an arsenic atom.

Use o to represent an electron from a hydrogen atom.
[2]

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 42
 (4 marks)

(b) Another hydride of arsenic has the composition below.

arsenic 97.4% hydrogen 2.6%

i) Calculate the empirical formula of this hydride from the above data.
Show your working.
[2]
ii) The mass of one mole of this hydride is 154 g. What is its molecular formula?
[1]
iii) Deduce the structural formula of this hydride.
[1]

(4 marks)

(c) In the 19th Century, a bright green pigment, copper(II) arsenate(V) was used to kill rats
and insects. In damp conditions, micro-organisms can act on this compound to produce
the very poisonous gas, arsine.

i) Suggest a reason why it is necessary to include the oxidation states in the name of
the compound.
[1]
ii) The formula for the arsenate(V) ion is AsO4 3–.

Complete the ionic equation for the formation of copper(II) arsenate(V).

......Cu2+ + ......AsO43– → ..................................

[2]

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 43
 (3 marks)

© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 44