Chemical Formulae, Equations, Calculations

Level IGCSE(9-1)

Subject Chemistry

Exam Board Edexcel IGCSE

Module Single Award (Paper 2C)

Topic Principles of Chemistry

Sub-Topic Booklet

Chemical Formulae, Equations, Calculations Model Answers 3

Time Allowed: 59 minutes

Score: /49

Percentage: /100

Grade Boundaries:

Chemical Formulae, Equations,

Calculations

Model Answers 3

Model Adel Adel Answer Keynswer Keynswer Key

Re Re

Red = And = And = Answerswerswer

1 The apparatus in the diagram was set up to demonstrate the rusting of iron.

(a) One week after the start of the experiment the volume of gas in the measuring cylinder has decreased.

After two weeks there is no further decrease in volume of gas in the measuring cylinder.

Explain these observations.

(2)

(b) Iron reacts with dilute sulfuric acid. The chemical equation for this reaction is Fe + H₂SO₄ĺ FeSO₄ + H₂

Complete the word equation for the reaction.

(2)

start after one week after two weeks iron wool measuring cylinder 100 cm3 air water 80 cm3 _{80 cm}3

This experiment is used to demonstrate the percentage of oxygen

contained within air.

The iron wool will react with the oxygen in the air

causing the water to travel up the inverted measuring cylinder. The water

will stop moving when all of the oxygen in the air has reacted.

hydrogen

. . . .

(c) Aqueous sodium hydroxide can be used to distinguish between solutions containing iron(II) ions (Fe2+_{) and iron(III) ions (Fe}3+_).

State the observation made when aqueous sodium hydroxide is added separately to each solution.

(2)

(Total for Question 1 = 6 marks)

Transition metals will form different coloured

precipitates with sodium hydroxide depending

on their valency.

Fe2+ produces a green precipitate Fe3+

produces a brown precipitate

2 This is a recipe for making Irish soda bread.

• add 170 g of wholemeal flour, 170 g of plain flour, 10 g of salt and 10.5 g of

bicarbonate of soda (sodium hydrogencarbonate, NaHCO₃) to a bowl and stir

• pour in 290 cm3 of buttermilk and stir quickly to form a soft dough • form the dough into a round ball and slightly flatten it

• cut a cross in the top and bake for 30 minutes in an oven at 200 °C

When sodium hydrogencarbonate is heated, it forms carbon dioxide gas. 2NaHCO₃ĺ Na₂CO₃ + H₂O + CO₂

(a) Calculate the mass, in grams, of carbon dioxide that would be produced by completely decomposing 10.5 g of sodium hydrogencarbonate.

M

[ _r of NaHCO₃ = 84] ₍₂₎

mass of carbon dioxide = . . . 2.8 g

First, you need to calculate the number of moles of sodium hydrogen

carbonate as this is what you can do easily with the data in the question:

Number of moles = mass ÷ relative formula mass

𝐍𝐮𝐦𝐛𝐞𝐫 𝐨𝐟 𝐦𝐨𝐥𝐞𝐬 = 𝟏𝟎. 𝟓 ÷ 𝟖𝟒 = 𝟎. 𝟏𝟐𝟓

The equation given in the question tells us that 2 moles of sodium hydrogen

carbonate will produce one mole of carbon dioxide. This means that there will

be 0.0625 moles of carbon dioxide produced from the 0.125 moles of sodium

hydrogen carbonate.

Now you can work out the mass of the carbon dioxide produced:

Mass = number of moles × relative formula mass

𝐌𝐚𝐬𝐬 = 𝟎. 𝟎𝟔𝟐𝟓 × 𝟒𝟒 = 𝟐. 𝟖𝐠

(b) Use your answer from part (a) to calculate the volume, in cm3_{, at room}

temperature and pressure, of carbon dioxide that would be produced by completely decomposing 10.5 g of sodium hydrogencarbonate.

Assume one mole of carbon dioxide has a volume of 24 000 cm3 _{at room}

temperature and pressure.

(2)

(Total for Question 2 = 4 marks)

volume of carbon dioxide = . . . cm3

Volume = number of moles of carbon dioxide × 24,000

𝐕𝐨𝐥𝐮𝐦𝐞 = 𝟎. 𝟎𝟔𝟐𝟓 × 𝟐𝟒, 𝟎𝟎𝟎

𝐕𝐨𝐥𝐮𝐦𝐞 = 𝟏𝟓𝟎𝟎𝐜𝐦𝟑

Exam Tip: VOLUME CALCULATIONS

If you are using this formula to complete the calculation,

you must ensure that you have converted the units from

dm3 to cm3. You can easily do this by multiplying by

24,000 rather than 24.

3 This question is about the reactions of compounds of antimony and phosphorus.

(a) Antimony (Sb) can be obtained from its oxide (Sb₂O₄) by heating it with carbon. The equation for this reaction is

Sb₂O₄(s) + 4C(s) ĺ 2Sb(s) + 4CO(g)

(i) Give the name of the gas produced in this reaction.

(1)

(ii) State why this gas is poisonous to humans.

(1)

Carbon monoxide.

CO is the gas produced.

(ii) What term is used to describe the energy required to start a reaction?

(1)

Activation energy

(Total for Question 3 = 5 marks)

(b) Phosphorus sulfide (P₄S₃) is one of the reactants used in match heads.

When a match is struck, energy is transferred to the reactants in the match head, starting a reaction.

(i) Balance the equation that represents this reaction. (2)

6KClO3(s) + S(s) + P4S3(s) KCl(s) + SO2(g) + P4O10(s)

When balancing an equation, you need to ensure that the numbers of all

atoms are the same on both sides of the equation.

There are 6 K atoms and 6 Cl atoms on the left hand side but only one of

each on the right hand side. Therefore a large 6 needs to be added in front of

the KCl on the right hand side.

6KClO3 + S + P4S3 → 6KCl + SO2 + P4O10

There are 4 sulfur atoms on the left hand side and only one the right hand

side. A large 4 needs to be added in front of the SO2 on the right hand side.

6KClO3 + S + P4S3 → 6KCl + 4SO2 + P4O10

4 Sodium azide (NaN3) is a stable compound at room temperature but decomposes when

heated to 300 °C. The equation for the decomposition is: 2NaN3(s) → 2Na(l) + 3N2(g)

Sodium azide is used to produce nitrogen gas to inflate car airbags.

If a car is involved in a collision, the sodium azide decomposes.

The nitrogen gas is produced very rapidly and the airbag inflates almost immediately.

(a) (i) A fully-inflated airbag has a total volume of 108 dm3_.

Calculate the amount of nitrogen, in moles, in a fully-inflated airbag.

[You should assume that the volume of one mole of nitrogen inside the airbag is 24 dm3_]

(2)

Amount of nitrogen = ... mol

108/24

=4.5

4.5

Divide the volume by

the molar volume

(24dm

_{). This is the}

volume that 1 mole of

gas occupies at room

temperature and

pressure.

(ii) Use your answer to (a)(i) to calculate the mass, in grams, of sodium azide required to produce 108 dm3 _{of nitrogen.}

(3)

Mass of sodium azide required = ... g

M

of NaN3 - 65

Moles of NaN

- 3 Or two thirds of (a)(i)

Mass of NaN

- 195 (g) Or moles of NaN

(Mark Consequentially at each stage)

Write down the molar

ratio and calculate the

number of moles of

NaN

required

Then convert this to

mass

2NaN

(s)

→

2Na(s) + 3N

(g)

Molar ratio is thus 2 : 2 : 3

Divide across by 3, so ⅔ moles of

NaN

produces 1 mole of N2

Therefore (⅔ x 4.5) = 3 moles of

NaN

is required

M

of NaN

= 65g

Therefore 3 x 65 = 195g NaN

is

required

195

(b) The airbag also contains potassium nitrate. This reacts with sodium formed in the decomposition of sodium azide. The equation for the reaction is:

10Na(l) + 2KNO3(s) → K2O(s) + 5Na2O(s) + N2(g)

(i) Suggest one reason why the makers of the airbag might want this reaction to

occur.

(1)

(ii) The airbag also contains silicon dioxide (SiO2) which reacts with the oxides

produced in the reaction above. This forms a glassy solid which seals all the products into the airbag.

The glassy solid contains potassium silicate (K2SiO3).

Construct an equation for the formation of potassium silicate from potassium oxide. Include state symbols.

(1)

Removes (harmful) sodium (Group I metal). The reaction also produces

more nitrogen gas and so the bag inflates more quickly.

K

O(s) + SiO

(s) -> K

SiO

(s)

Or

K

O(s) + SiO

(s) -> K

SiO

(I)

(c) Another use of sodium azide is to make lead(II) azide, which can be used as a detonator for explosives. Lead(II) azide has the formula of Pb(N3)2

Lead(II) azide can be made by the following reaction:

Pb(NO3)2(aq) + 2NaN3(aq) → Pb(N 3)2(s) + 2NaNO3(aq)

(i) What name is given to this type of reaction? (1)

(ii) What method would you use to remove the lead(II) azide from the final reaction mixture?

(1)

(Total for Question 4 = 9 marks)

This is a precipitation reaction or a double decomposition reaction.

The

state symbols indicate that a solid product and an aqueous product are

formed from both aqueous reactants, hence its a precipitation reaction.

Both the Pb(NO

) molecule and NaN

molecule decompose, hence the

term double decomposition.

5 A student set up this apparatus to measure the volume of carbon dioxide given off

when a sample of a carbonate of a Group 2 metal was reacted with dilute nitric acid.

She weighed out some of the carbonate and put it in a conical flask. She then added an excess of dilute nitric acid.

After adding the acid she placed the bung and delivery tube into the conical flask. She measured the total volume of gas collected at room temperature and pressure (rtp) in the measuring cylinder.

Her results are shown in the table.

Mass of Group 2 carbonate 0.888 g Volume of gas collected 144 cm3

The equation for the reaction is

XCO₃(s) + 2HNO₃(aq) o X(NO₃)₂(aq) + H₂O(l) + CO₂(g) where X is the symbol for the Group 2 metal.

water delivery tube

bung

dilute nitric acid

metal carbonate

250 cm3

measuring cylinder

(a) (i) Calculate the amount, in moles, of carbon dioxide gas collected.

(Assume that one mole of gas has a volume of 24 000 cm3 _{at rtp)} (2)

Amount of carbon dioxide gas collected = . . . mol

(ii) Deduce the amount, in moles, of the carbonate that reacted.

(1)

number of moles =

volume

24000

𝐧𝐧𝐧𝐧𝐧𝐧𝐧𝐧𝐧𝐧𝐧𝐧 𝐨𝐨𝐨𝐨 𝐧𝐧𝐨𝐨𝐦𝐦𝐧𝐧𝐦𝐦 =_{𝟐𝟐𝟏𝟏𝟐𝟐𝟐𝟐𝟐𝟐 = 𝟐𝟐. 𝟐𝟐𝟐𝟐𝟎𝟎}𝟏𝟏𝟏𝟏𝟏𝟏

Exam Tip: VOLUME CALCULATIONS

If you are using this formula to complete the calculation, you must ensure

that you have converted the units from dm3 to cm3. You can easily do this

by dividing by 24000 rather than 24.

= 𝟐𝟐. 𝟐𝟐𝟐𝟐𝟎𝟎

The equation shows that when 1 mole of carbonate reacts, one mole of carbon dioxide is

produced.

Therefore the number of moles of carbonate reacted is also 0.006 moles.

(iii) Using the mass of the carbonate and your answer to (a)(ii), calculate the relative formula mass (M_r) of this carbonate.

Give your answer to the nearest whole number.

(2)

Relative formula mass = . . . .

(iv) Calculate a value for the relative atomic mass of the Group 2 metal, X, and use the Periodic Table on page 2 to suggest its identity.

(3)

Relative atomic mass of X = . . . .

Identity of X = . . . .

relative formula mass =

mass

number of moles

𝐧𝐧𝐧𝐧𝐦𝐦𝐫𝐫𝐫𝐫𝐫𝐫𝐫𝐫𝐧𝐧 𝐨𝐨𝐨𝐨𝐧𝐧𝐧𝐧𝐧𝐧𝐦𝐦𝐫𝐫 𝐧𝐧𝐫𝐫𝐦𝐦𝐦𝐦 =𝟐𝟐. 𝟖𝟖𝟖𝟖𝟖𝟖_{𝟐𝟐. 𝟐𝟐𝟐𝟐𝟎𝟎 = 𝟏𝟏𝟏𝟏𝟖𝟖}

= 𝟏𝟏𝟏𝟏𝟖𝟖 = 𝟏𝟏𝟏𝟏𝟖𝟖

= 𝟏𝟏𝟏𝟏𝟖𝟖

Relative formula mass of CO3 = 12 + (16 x 3) = 60

RFM of metal carbonate – RFM of CO3 = RAM of metal

148 – 60 = 88

Metal = strontium

88

(b) After the student had completed the experiment she was told that the metal

(2)

(Total for Question 5 = 10 marks)

carbonate was calcium carbonate.

She calculated that 0.888 g of calcium carbonate would produce 213 cm3 _of

carbon dioxide.

She was certain that she had measured the mass of the metal carbonate correctly. Suggesttwo reasons why the volume of gas she collected was less than 213 cm3_.

Any two from:



gas was lost between adding acid and replacing the

bung



the bung does not fit or there are leaks in the

apparatus



some of the gas dissolved in the water



the carbonate was impure



the temperature of the gas was lower than room

6 Sulfur dioxide, SO2, is used as a preservative in wine.

The sulfur dioxide content of a wine can be found by titration. A chemist found that 25.0 cm3

of a sample of wine reacted with exactly 15.00 cm3 _{of 0.0010 mol/dm}3 _{aqueous iodine, I} 2(aq).

The equation for the reaction is

SO2(aq) + I2(aq) + 2H2O(l) → SO42–(aq) + 2I–(aq) + 4H+(aq)

(a) Calculate the amount, in moles, of iodine in 15.00 cm3 _{of a 0.0010 mol/dm}3 _solution.

(2)

(b) Deduce the amount, in moles, of sulfur dioxide in 25.0 cm3 _{of the wine. (1)}

number of moles = concentration x volume

1000

𝐧𝐧𝐯𝐯𝐚𝐚𝐧𝐧𝐫𝐫𝐫𝐫 𝐚𝐚𝐨𝐨 𝐚𝐚𝐚𝐚𝐫𝐫𝐫𝐫𝐦𝐦 =𝟖𝟖.𝟓𝟓 𝐱𝐱 𝟓𝟓.𝟓𝟓𝟓𝟓𝟖𝟖𝟓𝟓_{𝟖𝟖𝟓𝟓𝟓𝟓𝟓𝟓} = 𝟖𝟖. 𝟓𝟓𝐱𝐱𝟖𝟖𝟓𝟓-5

Exam Tip: UNITS

This formula gives volume in dm3 but the question gives volume in

cm3. You must therefore convert your units by dividing by 1000.

The equation shows us that one mole of sulfur dioxide reacts with one

mole of iodine. Therefore if there are 1.5 x10-5 moles of iodine,

there

will be 1.5 x10-5 moles of sulfur dioxide.

(c) Calculate the concentration, in mol/dm3_{, of sulfur dioxide in the wine.}

(2)

(d) Calculate the concentration, in g/dm3_{, of sulfur dioxide in the wine. (2)}

(e) A concentration of sulfur dioxide that is greater than 0.16 g/dm3 _{makes wine}

(1)

concentration = number of moles x 1000

volume

𝐚𝐚𝐚𝐚𝐧𝐧𝐚𝐚𝐫𝐫𝐧𝐧𝐫𝐫𝐫𝐫𝐫𝐫𝐫𝐫𝐫𝐫𝐚𝐚𝐧𝐧 = 𝟓𝟓.𝟓𝟓𝟓𝟓𝟓𝟓𝟓𝟓𝟖𝟖𝟓𝟓 𝐱𝐱 𝟖𝟖𝟓𝟓𝟓𝟓𝟓𝟓_{𝟐𝟐𝟓𝟓} = 𝟓𝟓. 𝟓𝟓𝟓𝟓𝟓𝟓𝟎𝟎 mol/dm3

g/dm3 = mol/dm3 x relative formula mass

g/dm3 = 0.0006 x 64 = 0.038 g/dm3

unpleasant to drink.

Use the value you have calculated in (d) to state whether the wine is drinkable.

7 (a) The list shows some techniques used to separate mixtures. A B C D E crystallisation filtration fractional distillation paper chromatography simple distillation

Complete the table to show the best method of obtaining each substance from the mixture.

In each case, choose one of the letters A, B, C, D or E. Each letter may be used once, more than once or not at all.

(4)

Substance Mixture Letter

sand sand and water

solid copper(II) sulfate aqueous copper(II) sulfate red food dye mixture of food dyes

kerosene crude oil

C

B

A

D

Sand is an insoluble solid so can be separated from water using filtration. The

water will pass through the filter and the sand will be left in the filter paper.

In order to obtain solid copper sulfate from aqueous copper sulfate you must

evaporate off the water from the solution. Crystallisation is the process by which

crystals are formed from a solution.

Different food dyes will have different solubilities in solvents such as water. This

means we can separate them using chromatography.

Fractional distillation utilises the boiling points of liquids to separate them from

mixtures. Crude oil is a mixture of hydrocarbons. When heated, the hydrocarbons

can be separated into fractions because they all have different boiling points.

(b) Gold occurs in ores, which are mixtures of gold and other substances. Several elements and compounds are used in the extraction of gold from its ores.

Each box below represents the substances present in one part of the extraction process. Classify the contents of each box as a compound, an element or a mixture by writing your choice below each box.

(3)