24 Section 3 - Equations, Calculations and Electrolysis

1 Carbon monoxide can form in poorly-ventilated gas fires.

Below is an equation for this reaction, which is incorrectly balanced.

c + 02 ~ co
Which one of the following equations is balanced correctly?

D c 2C + 02 ~ 2CO

D B c + 02 ~ 2CO D D c + 202 ~ 4CO

[Total] mark]

2 Methane (CH4) burns in oxygen (0) to make carbon dioxide (C02) and water (H20).

a) Write a word equation for this reaction.

[1}

b) Write a balanced chemical equation for this reaction.

[2}
[Total 3 marks}

3 A more reactive halogen can displace a less reactive halogen from a solution of its salt.

Write a balanced chemical equation for the reaction of chlorine (Clz) with potassium bromide
(KBr). The products of this reaction are bromine (Br) and potassium chloride (KCl).

[Total 2 marks}

4 Acids can react with a variety of different metals and their oxides.

a) Balance the following chemical equations.

\\I I I I I I I I I I I I I I I I I I I I I I;
i) ........ HCl + ........ CuO ~ ........ CuC12 + ........ H20 := Not every compound will ~
= need a number in front of =
=. it to balance the equations. ~
ii) ........ HN03 + ........ MgO ~ ........ Mg(N03) 2 + ........ H 20 /If I I I I I I I I I I I I I I I I I I I I 1\

[2}

b) Write a balanced chemical equation for the reaction ofhydrochloric acid (HCI)
with aluminium (AI) that produces aluminium chloride (AlCl) and hydrogen (Hz).

[2}
[Total 4 marks}

Section 3 - Equations, Calculations and Electrolysis

 25
5 Reduction reactions involve the removal of oxygen from a compound.

a) Balance the following equation, which shows the reduction of copper oxide .

........ CuO + ........ C ~ ........ Cu + ........ C02

[1]

b) Balance the following equation, which shows the reduction of iron oxide .

........ CO + ........ Fe2 0 3 ~ ••...•.• C0 2 + ........ Fe

[1]
[Total2 marks]

6 Sodium (Na) is a reactive alkali metal, which is found in Group 1 of the periodic table.

a) Sodium reacts with chlorine (Cl) to form sodium chloride (NaCl).

Write a balanced chemical equation for this reaction.

[2]

b) When a solution of calcium hydroxide (Ca(OH) 2) is mixed with solid sodium carbonate (Na2CO),
sodium hydroxide solution (NaOH) and a precipitate of calcium carbonate (CaC0 3) are produced.
The equation for the reaction is:

Ca(OH)l ....... ) + Na2 C0 3 ( •.•.•..• ) ~ 2NaOH( ........ ) + CaC03 ( ........ )

Complete the equation by adding state symbols.

[1]

c) Solid sodium metal reacts with water to form a solution of sodium hydroxide (NaOH).
Hydrogen (H2) is also given off.
Write a balanced chemical equation for this reaction, including state symbols.

[3]
[Total 6 marks]

7 Balance the following symbol equation to show how sulfur reacts with nitric acid .

........ S + ........ HN03 ~ .....•.. H 2 S04 + ........ N0 2 + ........ H 2 0

[Total 1 mark]

Exam Practice Tip Score

It's important that you get to grips with balancing equations because it often features in exam papers.
Remember to double check your equation after you've balanced it and you'll be on to a winner. You need D
to make sure you know your state symbols too, because they come hand-in-hand with chemical equations. 19
©D Section 3 - Equations, Calculations and Electrolysis
26

Relative Formula Mass

1 Which ofthe following compounds has a relative formula mass of62?

D A sodium chloride, NaCl 11 11

exam which you can look =
~;
I I I I I I I I I I I I I I I I I I I I I I I I I I II;/

D B potassium bromide, KBr =, 1 11 1 1 1 1 1 1 I I II I 1

- at any time. There s one

l

In
.
You'll have a peno,dlc ta. e ~~yob:ok on the inside back cover. =
IS =
. .

~ t~e1 ~~~~t;~~s 1 ~~ 1t~: 1 \~~~ 1 1 1\-\

5

D C magnesium fluoride, MgF 2

You might fmd it useful for
/;I\ I I I I I\ I I I I I I I I I I I I I I\\ I I

D D sodium bromide, NaBr

[Total 1 mark]

2 A solution of calcium hydroxide, Ca(OH)z can be known as limewater.

Calculate the relative formula mass of calcium hydroxide, Ca(OH)2 .

relative formula mass = ......................................... .

[Total 2 marks]

3 The formula of the compound zinc cyanide is Zn(CN)2 .

Calculate the relative formula mass of zinc cyanide.

relative formula mass = ......................................... .

[Total 2 marks]

4 The equation below shows a reaction between an unknown element, A, and water.
The total Mr of the products is 114.

Identify element A.

Element A= ..................................... .
[Total 3 marks]

Score: D
8
Section 3 - Equations, Calculations and Electrolysis ®D ©D ©D
 27

I . ..
OZ1i'ic:;;a&2L.,,oSF~;;:-""''".2 M=-"'!JA2iliiE·mr_,i!E'i;&"'"'-~::u±:?;:;;t:J!t::;s:s!i\i&.~'~~:e:a:;;::_,;im%,.,.Uf!BD ·=...;;:tnni¥.
Moles ·.. ·. ·.. .· . · J
ili£M£iii~.;t,S!..¥:JiS1liG!!M!@iJC-_ffilii'~>&S_@'if'J!i'&\Q&"'-20!'*",~;u:.,._13PJ5'1f&!li;m;;,_"'2Q""""'""-"""--4c&X---::iliD:;r;;;;;m.;_.dl}_:fuG!i!V\£1£,__~\5S__,__3ii£1-""'"i"i2&<illi!fS:Ji.~

1 A scientist measured out one mole of iron into a beaker.

What is the mass of one mole of iron?

D A 56g D B 26kg D c 26g D D 56kg

[Total1 mark}

2 Calculate the number of moles in 9. 0 g of carbon.

The relative atomic mass, A r , of C = 12.

number of moles = .......................... .

[Total 1 mark]

3 A pharmacist is synthesising aspirin, C9H 80 4 , as part of a drugs trial.

After the experiment, the pharmacist calculates that she has made
12.4 moles of aspirin. What mass of aspirin has the pharmacist made?
The relative atomic mass, A r , of C = 12, of H = 1 and of 0 = 16.

mass= ........................... g
[Total 2 marks}

4 A scientist finds a sample vial whilst clearing out a cupboard in the lab.
The label on the vial says that it contains 0.075 moles of an unknown metal oxide.

a) The sample weighs 3.0 g. Calculate the Mr of the metal oxide.

Mr= ........................... g
[1}

b) Which of these compounds could be the metal oxide?

DB MgO DC CaO
[1}
[Total 2 marks}

Section 3 - Equations, Calculations and Electrolysis

28

5 A student was asked to calculate the number of moles and the masses
of different compounds she would be using in her lab practical.

a) Calculate the number of moles in 275 g of zinc carbonate (ZnCOJ

number of moles = .......................... .

[21

b) Calculate the mass of 6 moles of hydrochloric acid (HCl).

mass= ........................... g
[21
[Total 4 marks1

6 In a reaction between iron and oxygen, 3.52 kg of iron oxide

is formed. The equation for the reaction is shown below.

4Fe + 302 ~ 2Fe2 0 3

Calculate the number of moles of iron oxide formed in this reaction.

number of moles = .......................... .

[Total 3 marks1

7 A teacher has a 140 g sample of potassium hydroxide (KOH).

Calculate how many more grams of KOH the teacher needs to have a 4 mole sample.

Extra KOH needed= ........................... g

[Total 2 marks1

Score: D
15
Section 3 -Equations, Calculations and Electrolysis ®D ©D ©D
 29

1 A student is investigating the combustion of metals.

a;...@
":§}. .
a) The student burns 12 g of magnesium in air to produce magnesium oxide (MgO).
2Mg + 0 2 ~ 2Mg0
Calculate the maximum mass of magnesium oxide that could be produced in the reaction.

Mass of magnesium oxide = ........... 00 . . . . . . g

[31
b) Using the chemical equation below, work out the mass of sodium
that the student would need to burn in order to produce 6.2 g of sodium oxide.

Mass of sodium = ................... g

[31
[Total 6 marks1

2 Aluminium and iron oxide (Fe 20 3) react to produce aluminium oxide (Al20) and iron.

2Al + Fe2 0 3 ~ Al 20 3 + 2Fe

a) What is the maximum mass of iron that can be produced from 20 g of iron oxide?

Mass of iron = ................... g

[31
b) What is the maximum mass of aluminium that will react with 32.0 kg of iron oxide?

,I II I I I I I I I I I I I I I I I I I I I I I I I II--:_
:: Don't get caught out by -
_ changes in the units - part a) _
:: was in g, but part b) is in kg. :;:
//11 11 II 1 II I II i l l I I II I I l l I I I I

Mass of aluminium = .. 00 . . . . . . . . . 00 . . . . kg
[31
[Total 6 marks1

Section 3 - Equations, Calculations and Electrolysis

30

3 Sodium sulfate (Na2S04 ) is made by reacting sodium hydroxide (NaOH)

with sulfuric acid (H2SO4). Water is another product of this reaction.

2NaOH + H 2 S04 ~ Na2 S04 + 2H2 0

a) What mass of sodium hydroxide is needed to make 71 g of sodium sulfate?

Mass of sodium hydroxide = ................... g

[31
b) What is the maximum mass of water that can be formed when 24.5 g of sulfuric acid
reacts with sodium hydroxide?

Mass of water = ................... g

[31
[Total 6 marks1

4 Iron oxide is reduced to iron inside a blast furnace using carbon.

There are three stages involved. The equations for these three stages are shown below.

Stage 1: CcsJ + 0 2(gJ ~ C02(gJ

Stage 2: C02(gJ + CcsJ ~ 2CO(gJ
Stage 3: 3CO(gJ + Fe2 0 3csJ ~ 3C02Cgl + 2Fe 0l
If 10.2 g of carbon are used in stage 2, and all the carbon monoxide produced gets used in stage 3,
what mass of C02 is produced in stage 3?

Mass of C02 = ................... g

[Total 5 marks1

Exam Practice Tip Score

It's really important to get your head around the method for calculating masses in reactions. It's just a case
of finding the relative molecular masses for the bits that you want, and then doing a spot of dividing and
multiplying. And don't forget to double check your working to make sure your answer is right. 23

Section 3 - Equations, Calculations and Electrolysis ®D ©D ©D

 31

J
1 A teacher wanted to produce some silver chloride (Agel). The teacher added a
carefully measured mass of silver nitrate to an excess of dilute hydrochloric acid.
1.2 g of silver chloride were produced.

a) Explain what is meant by the yield of a chemical reaction.

[1}

b) The teacher calculated that he should get 1. 6 g of silver chloride from the reaction.
What was the percentage yield?

Percentage yield = ................... %

[1}
[Total2 marks]

2 Solutions of barium chloride and sodium sulfate were mixed together in a beaker
to produce barium sulfate. The solution was filtered to obtain the solid barium sulfate.

The reaction was predicted to give a yield of 15 g of barium sulfate. However, after the
experiment was completed a yield of only 6 g had been obtained. Calculate the percentage yield.

Percentage yield = ................... %

[Total] mark]

3 When heated, calcium carbonate decomposes to form calcium oxide and carbon dioxide.
The equation for the reaction is: CaC0 3 ---+ CaO + C02

In an industrial reaction, 68.00 kg of calcium carbonate decomposed to form

28.56 kg of calcium oxide, CaO. Calculate the percentage yield of calcium oxide.

Percentage yield = ................... %

[Total 5 marks}

Score: D
8
Section 3 - Equations, Calculations and Electrolysis
32

1 Nitrogen monoxide, NO, reacts with oxygen, 0 2, to form oxide R.

A 100 g sample of oxide R contains 30.4 g of nitrogen and 69.6 g of oxygen.

Work out the empirical formula of oxide R.

Empirical formula = .................................. .

[Total 3 marks]

2 A compound contains 10.1% aluminium and 89.9% bromine by mass.

Calculate the empirical formula of the compound.

Empirical formula = .................................. .

[Total 3 marks]

3 1.48 g of a calcium compound contains 0.8 g calcium

and 0.64 g of oxygen. The rest is hydrogen.

Work out the empirical formula of the compound.

Empirical formula = .................................. .

[Total4 marks]

4 A molecule has an empirical formula of C3H 70, and a relative molecular mass of 118.

Deduce the molecular formula of the molecule.

Molecular formula = .................................. .

[Total 3 marks]

Score: D
13
Section 3 - Equations, Calculations and Electrolysis ®D: ©0; ©D
 33

Finding Fozmulae Using periments

1 A student is carrying out a reaction to find the formula of an oxide
of manganese. His experimental set-up is shown on the right.

a) What is the name given to this type of reaction?

D A neutralisation
D B combustion
oxide of
D C reduction manganese

D D polymerisation
[I]
gas

Bunsen burner
b) During the reaction, manganese and oxygen are produced.
Some of the student's results are shown in the table .
.•· Itt~§s.pftest tube an4 bung . ·· 36.48 g
84.88 g
i) Calculate the mass of manganese present in the sample.

mass of manganese = ....................................... g

[I}

ii) The mass of oxygen present was 14.08 g. Calculate the empirical formula of the oxide.
A/Mn) =55, AiO) = 16

empirical formula = ...................................... .

[3]
[Total 5 marks]

2 A student carried out an experiment to calculate the empirical formula

of an oxide of iron. She bumt 3. 808 g of iron until it had all reacted.
She found that the mass of the product was 5.440 g.

a) Suggest a piece of equipment that the student could use to carry out the reaction in.

[I]

b) Calculate the empirical formula of the iron oxide formed during the experiment.
A/Fe) =56, Ar(O) = 16

empirical formula = ...................................... .

[4]
[Total 5 marks]

Score: D
10
®D ©D· ©D Section 3 -Equations, Calculations and Electrolysis
 34

I
Water of Crystallisation
1 Na2 C0 3 .xH2 0 is a hydrated salt, which means that water molecules are
present in the lattice structure. This water is called water of crystallisation.

By heating a sample of a hydrated salt you can gradually remove the water of crystallisation to
form an anhydrous salt. This can be done by placing a sample of the hydrated salt into a crucible
and gently heating it using a Bunsen burner.
A student used this method to remove the water of crystallisation from Na2 C0 3 .xH2 0 to produce
the anhydrous salt Na2C0 3 . The student put a sample ofNa2 C0 3 .xH2 0 into a crucible, weighed
both together, and recorded the mass. The student then heated the sample for 2 minutes, left it to
cool and recorded the mass of the sample and crucible again. The student repeated this until two
separate masses that had the same value were recorded. The student made a note of this mass.

Starting mass of crucible+ Na2 C0 3 .xH2 0 61.224 g

The student recorded
the following masses: Final mass of crucible+ Na2 C0 3 56.364 g

a) What is the purpose of heating the hydrated salt until the mass remains constant?

[1}

b) The mass ofthe crucible was 53.500 g.

Use this to calculate the masses of the following samples.
i) Na2 C0 3 .xH20

[1}

[1}

c) Use your answers to part b) to work out the value ofx in Na2 C0 3 .xH2 0.
(Mr H 2 0 = 18)

X= ....................... ..
[4}
[Total 7 marks}

Score: D
7
Section 3 - Equations, Calculations and Electrolysis ®D ©D; ©D
 36

Moles and Concentration

1 A student had a solution of an alkali with an unknown concentration.
He also had some hydrochloric acid (HCl) at a concentration of 1.00 moVdm3 •

a) The student carried out an experiment to find out how much hydrochloric acid was
needed to neutralise the alkali. He did the experiment 3 times and his results are shown
in the table below. Calculate the mean volume of hydrochloric acid that was needed.

,:r:E~:e!lfit~nf'Numl)er '· ···

i •.• 2 3
12.55 12.45

Mean volume = ......................... cm3

[21

b) Use your answer to part a) to calculate the number of moles of hydrochloric acid that reacted.

Number of moles = ........................ .

[21
[Total 4 marks1
N
...
"'~
~ 2 Sodium hydroxide (NaOH) reacts with sulfuric acid
to produce sodium sulfate (Na2S04) and water.

a) How many moles of sodium hydroxide are in 120 cm3 of a 2.5 mol/dm3 solution?

Number of moles = ........................ .

[11

b) What is the concentration of a solution with 3 moles of sodium sulfate in 750 cm3 ?

Concentration= ......................... mol/dm3

[11

c) Give your answer for part b) in g/dm3 .

Concentration= ......................... g/dm3

[21
[Total 4 marks1

Score: D
8
®D Section 3 - Equations, Calculations and Electrolysis
 36

I Calculating Volume~ ·
1 The volume that one mole of a gas occupies is called its molar volume.

a) State the value of the molar volume of a gas at room temperature and pressure.

[1]

b) What volume does 1.5 moles of hydrogen take up at room temperature and pressure?

Volume = ........................... dm3

[1]
[Total 2 marks]

2 How many moles are there in 2250 cm3 of ammonia (NH3)?

Number of moles = .......................... .

[Total] mark]

1"1

~
~
3 A scientist completely burns 7.8 g of nitric oxide in oxygen.
The equation for the reaction is: 2NO + 0 2 ~ 2N0 2

a) Calculate the volume of nitrogen dioxide produced (at room temperature and pressure).

Volume = ........................... dm3

[3]

b) Find the volume of oxygen that reacted (at room temperature and pressure).

Volume = ........................... dm3

[2]
[Total 5 marks]

Score: D
8
Section 3 - Equations, Calculations and Electrolysis ®D ©D ©D;