AQA A Level Chemistry

Topic Questions

1.2 Formulae, Equations & Calculations

1a2 marks

Define the term relative atomic mass, Ar, of an element.

1b3 marks

Metal carbonates have a variety of medicinal applications.

Magnesium and calcium carbonate are often used to treat heartburn and indigestion. Iron carbonate is used as a dietary supplement to treat anaemia. Lithium carbonate can be used in the treatment of mental health illnesses.

Write a balanced symbol equation, including state symbols, for the reaction of calcium carbonate with hydrochloric acid to form calcium chloride and two other products.

1c3 marks

A student wants to find the mass of magnesium carbonate in a 500 mg indigestion tablet. She crushes the tablet and reacts it with 25 cm3 of 1.00 mol dm-3 sulfuric acid according to the following equation:

MgCO3 + H2SO4  →  MgSO4 + H2O + CO2 

i)
Calculate the number of moles of magnesium carbonate in the tablet.  
You can assume that the magnesium carbonate is responsible for all of the tablet's mass.

ii)
Calculate the number of moles of sulfuric acid that the tablet reacts with.

iii)
Explain how your answers to parts (i) and (ii) show that the sulfuric acid is in excess.
1d3 marks

The student then titrated the excess sulphuric acid with 0.5 mol dm3 sodium hydroxide solution. The student’s titration results are shown in Table 1.

Table 1

Titration number

Initial volume / cm3

Final volume / cm3

Titre / cm3 

Rough

0.00

21.70

21.70

1

21.70

43.05

 

2

0.10

21.60

 

3

21.60

43.00

 

4

0.05

21.40

 

i)
Complete the table by calculating the titres.

ii)
Calculate the average titre using concordant results.

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2a1 mark

State the meaning of the term empirical formula.

2b2 marks

An unknown compound contains carbon, hydrogen and oxygen only. It was shown to contain 3.20 g carbon, 0.54 g hydrogen and 4.26 g oxygen.

Calculate the empirical formula of the unknown compound.

2c4 marks

A student performed a titration to determine the concentration of ethanoic acid in a sample of vinegar, by using the following method:

  1. Measure 25 cm3 of vinegar into a conical flask using a volumetric pipette.
  2. Add 4-5 drops of phenolphthalein indicator to the conical flask.
  3. Fill the burette with 0.1 mol dm-3 sodium hydroxide solution.
  4. Ensure that the funnel is removed from the burette and that the jet-space is filled before starting titration.
  5. Take the initial reading of the burette.
  6. Add the sodium hydroxide solution to the conical flask, with mixing, until there is a permanent colour change in the indicator.
  7. Record the final volume of the burette.

The student’s results are recorded in Table 1.

Table 1

Titration number

Initial volume / cm3

Final volume / cm3

Rough

0.00

20.10

1

0.00

19.55

2

0.10

19.40

3

19.40

39.10

The student completes the following calculation for the average titre using their results;

Average titre = fraction numerator 19.55 space plus space 19.40 over denominator 3 end fraction = 12.98 cm3

i)
Identify the errors that the student has made in their calculation.

ii)
Describe what the student should do to get an accurate average titre.
2d2 marks

Another student completes titration to determine the concentration of ethanoic acid in the vinegar sample with 0.10 mol dm-3 sodium hydroxide solution. Their average titre was calculated to be 19.35 cm3.

i)
Calculate the number of moles of sodium hydroxide used in the titration.

ii)
The equation for the reaction between the ethanoic acid in the vinegar and the sodium hydroxide is:

CH3COOH (aq) + NaOH (aq)  →  CH3COONa (aq) + H2O (l)

Use your answer to part (i), to calculate the concentration of ethanoic acid in the vinegar sample.

(If you did not achieve a value for the number of moles of sodium hydroxide, assume that this is 0.0018 moles. This is not the correct answer.)

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3a2 marks

Define the term relative molecular mass, Mr, of a molecule.

3b4 marks

Despite having poor atom economy, iron is still produced in the blast furnace by the reduction of iron oxide with carbon monoxide.

i)
Balance the equation for the production of iron in the blast furnace.

   ….Fe2O3 + ….CO  →  ….Fe + ….CO2

ii)
By calculating the atom economy for the production of iron, show that this reaction has poor atom economy. 
Give your answer to the appropriate number of significant figures.
3c3 marks

Cyclohexene reacts with bromine to form 1,2-dibromocyclohexane, C6H10Br2.

Show, by calculation, that this reaction has a high atom economy.

3d2 marks

There are a number of ways to prepare magnesium sulphate:

  1. Mg + H2SO4 → MgSO4 + H2 
  2. MgO + H2SO4 → MgSO4 + H2
  3. MgCO3 + H2SO4 → MgSO4 + CO2 + H2O

Explain which of the reactions has the highest atom economy.

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4a2 marks

Define the term relative formula mass of an ionic compound.

4b3 marks

12.00 g of a metal oxide contains 10.66 g of copper and the remainder of the compound is oxygen.

What is the empirical formula of the metal oxide?

4c1 mark

Magnesium oxide is formed by the combustion of magnesium.

Write the ionic equation for the formation of the magnesium oxide product.

4d3 marks

A student started with a 3.51 g piece of magnesium ribbon. They placed the magnesium ribbon inside a crucible and heated the magnesium directly to make magnesium oxide.

i)
Calculate the number of moles of magnesium used in the reaction.

ii)
Calculate the theoretical mass of magnesium oxide that can be produced in the reaction.

iii)
The student actually produced 4.11g of magnesium oxide. Calculate the percentage yield of their reaction.

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5a3 marks

Calcium carbonate undergoes thermal decomposition to form two products.

i)
Write the balanced symbol equation for this reaction.

ii)
Describe a test to identify the gas produced in the reaction and state the expected observation.
5b2 marks

A student heated a 3.00 g sample of calcium carbonate inside a crucible and recorded the mass every 30 seconds.

The results of the student’s experiment are shown in Table 1.

Table 1

Time / s

0

30

60

90

120

150

180

210

Mass / g

3.00

2.45

2.37

2.20

2.05

1.94

1.85

1.78

Plot these results on the grid in Figure 1 and draw a line of best fit. The first two points have been plotted.

Figure 1

14

5c5 marks
i)
Calculate the theoretical mass of calcium oxide that the student could obtain from the thermal decomposition of 3 g of calcium carbonate.

ii)
Calculate the percentage yield of calcium oxide for the student’s experiment.
Use the relevant information in part b.

iii)
Suggest why the student’s percentage yield was not 100%.
5d2 marks

The student repeated the experiment using a different method, collecting the gaseous product.

3.00 g of calcium carbonate was placed into a boiling tube and heated directly.

Draw a labelled diagram to show how you would set the apparatus up for this experiment.

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1a3 marks

A hydrated salt, R, has the formula Na2CO3.xH2O. 

Describe the chemical tests, including reagents and observations, that could be performed on the hydrated salt, R, to show that it is a sodium carbonate salt.

1b5 marks

A sample of the hydrated sodium carbonate salt, R, was heated to a constant mass in a 92.200 g evaporating basin. Mass measurements were taken every five minutes and are shown in Table 1

Table 1

Time / minutes

Mass of evaporating basin and hydrated sample / g

0

92.700

1

92.514

2

92.412

3

92.412

4

92.412

Determine the value of x in Na2CO3.xH2O for the hydrated salt, R.

1c3 marks

Explain the effect that not heating the salt to a constant mass will have on the value of x in Na2CO3.xH2O.

1d3 marks

Another hydrated salt, S, was analysed and found to have the following percentage composition by mass:

16.98% Ca; 11.86% N; 67.77% O; 3.39% H

Determine the empirical formula of the compound, showing clearly that it is a hydrated salt. 

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2a4 marks

A student completed an investigation to identify the metal in an unknown metal bicarbonate, MHCO3. They dissolved 1.94 g of the unknown metal bicarbonate into solution and reacted it with hydrochloric acid, under standard conditions. The volume of
carbon dioxide released was recorded every minute.

The student’s results are shown in Table 1.

Table 1

Time / mins

0

1

2

3

4

5

6

7

8

Volume of CO2 produced / cm3

0

138

262

363

433

471

480

480

480

Plot these results on the grid in Figure 1 and draw a line of best fit.

Figure 1

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2b4 marks

The metal bicarbonate reacts with hydrochloric acid according to the following equation.

MHCO3 (aq) + HCl (aq) → MCl (aq) + CO2 (g) + H2O (l) 

Use the information in part (a) to calculate the number of moles of metal bicarbonate.

The gas constant, R, = 8.31 J K-1 mol-1.

Record your answer to the appropriate number of significant figures.

2c2 marks

Use the information from parts a and b to calculate the Mr of the metal bicarbonate.

2d2 marks

Use your answer from part (c) to deduce the identity of the metal in the unknown metal bicarbonate, MHCO3.


(If you have been unable to calculate an answer for part (c), you may assume an Mr for the unknown metal bicarbonate of 194. This is not the correct answer.)

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3a5 marks

Succinic acid is a diprotic acid as shown in Figure 1.

Figure 1

4A student performs an investigation to determine the structural formula of succinic acid. The equation for the neutralisation of succinic acid, represented as H2A, with sodium hydroxide is:

H2A (aq) + 2 NaOH (aq) → Na2A (aq) + 2 H2O (l) 

  1. A student prepared a 250 cm3 standard solution using 1.42 g of succinic acid.
  2. They transferred 25.0 cm3 of the succinic acid solution to a conical flask.
  3. The succinic acid solution was titrated against 0.10 mol dm−3 sodium hydroxide solution.
  4. Their titration results are shown in Table 1.

Table 1

Titration

Rough

1

2

3

Final volume / cm3 

23.80

24.10

47.90

25.20

Initial volume / cm3 

0.00

0.00

24.10

1.00

Titre / cm3 

       

Use the results to calculate the number of moles of succinic acid in the standard solution.

3b3 marks

Using your answer to part (a), calculate the Mr of the succinic acid sample.

Deduce the value of n in HOOC(CH2)nCOOH to the nearest whole number and state the structural formula for the succinic acid.

3c2 marks

At the start of the rough titration, the student had not removed the funnel from the burette. The funnel was removed before the final rough titration reading was taken.

Explain what effect, if any, this will have on the final reading of the rough titration.

3d2 marks

The burette used by the student for their titration has an error of ±0.05 cm3. It was used to determine the initial and final reading of the titration.

Calculate the maximum percentage uncertainty in using the burette, using an average titre of 23.70 cm3. Note, this is not the answer for part (a).

Give your answer to 2 decimal places.

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4a7 marks

A student investigates the mass of calcium carbonate in an indigestion tablet by titration.

Three indigestion tablets were reacted in a beaker containing 50.0 cm3 of 1.00 mol dm-3 hydrochloric acid. 

i)
Describe how the resulting solution is then used to make a 250 cm3 standard solution.

ii)
Estimate the maximum volume of liquid added to make the standard solution and justify your answer.
4b7 marks

The student titrates 25.0 cm3 of the standard solution from part (a) with 0.10 mol dm-3 sodium hydroxide solution. The average titre was found to be 24.90 cm3.

CaCO3 (s) + 2 HCl (aq) → CaCl2 (aq) + H2O (l) + CO2 (g) 

HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l) 

Calculate the mass of calcium carbonate (Mr = 100.1), in mg, present in one indigestion tablet. Give your answer to the appropriate number of significant figures. 

4c4 marks
i)
During one of the titrations, the student noticed an air bubble in the jet space of the burette.
Explain the effect, if any, this air bubble has on the final burette reading.

ii)
During the titrations, the student washes the sides of the conical flask with deionised water.
Explain the effect, if any, rinsing with deionised water has on the burette readings.
4d2 marks

The maximum error for both the pipette and the burette used in the student’s titration was ± 0.05 cm3

Use information from part b) and an average titre value of 24.50 cm3 to justify which piece of equipment has the lowest percentage uncertainty in its measurement.

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5a3 marks

4.25 g of an unknown metallic element, Z, reacts completely with 1000 cm3 of oxygen at 100 kPa according to the following equation:

2Z + O2 → 2ZO

The volume of one mole of gas in these reaction conditions is 24.0 dm3.

Determine the identity of the unknown metallic element, Z.

5b6 marks

An unknown acid has the empirical formula CH2O and a density of 1.05 g cm-3.

1.71 cm3 of this acid is shown to combust completely with 1486 cm3 of oxygen.

Write the balanced symbol equation, including state symbols, for the reaction of 0.03 moles of this unknown acid with oxygen under standard conditions.

The gas constant, R, = 8.31 J K-1 mol-1.

5c3 marks

A Group 2 metal sulfate, XSO4, undergoes thermal decomposition to produce the metal oxide, XO, and two other gaseous products with an atom economy of 56.52%. The metal sulfate is known to be sparingly soluble in water.

Using a balanced symbol equation, justify if the metal, X, is strontium.

5d2 marks

Explain the effect that increasing the temperature for the thermal decomposition reaction in part (c) will have on the percentage atom economy for the reaction.

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1a3 marks

A synthetic carbohydrate was isotopically labelled with carbon-13.

Analysis of a 12 g sample of the carbohydrate found it to contain 6.93 g carbon, 0.80 g hydrogen and the remainder was oxygen.

Determine the empirical formulae of the carbohydrate.

1b1 mark

The fermentation of glucose, C6H12O6, is one method scientists can use to produce ethanol.  

Write a balanced symbol equation for this reaction. 

1c3 marks

During the fermentation process in the reaction in part (b), 312 g of ethanol was produced from 1.5 kg of glucose. 

Calculate the percentage yield of this reaction to 1 decimal place. If you were unable to produce an equation for part (b), assume that 1 mole of glucose would produce 4 moles of ethanol for this question (this is not the correct molar ratio). 

1d4 marks

Ethanol can also be produced by the hydration of ethene, as shown in the following equation: 

C2H4 (g) + H2O (g)  →  C2H5OH (g)

A student predicts that the production of ethanol from the fermentation of glucose will have a higher atom economy than from the hydration of ethene. 

i)
Is the student correct? Justify your answer.

ii)
State one advantage in industry of developing a chemical process which has a high atom economy. This advantage can be ethical, environmental or economical.

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2a5 marks

Concrete is one of the most common building materials and also one of the top three producers of carbon dioxide in the world. One of the main components of concrete is cement which is made by the thermal decomposition of limestone with other materials
such as clay in a kiln at temperatures around 1450 degreeC.

Calculate the mass, in kg, of carbon dioxide produced when 1 tonne of limestone is thermally decomposed.

2b4 marks

‘Asbestos’ is a generic, commercial term used to describe a number of mineral fibres, which were once commonly found in most buildings. The most common type of asbestos, chrysotile, makes up the vast majority of all asbestos products. Its chemical composition is 28.03% magnesium, 21.60% silicon, 1.16% hydrogen and the rest is oxygen. 

Calculate the empirical formula for chrysotile asbestos.

2c2 marks

The Mr of chrysotile is 520.2. 

Use your answer to part (b) to calculate the actual molecular formula of chrysotile.

2d2 marks

Another industrial mineral, belite, is used in the manufacture of cement. It’s main component is dicalcium silicate, Ca2SiO4, which reacts with water to form Ca3Si2O4(OH)6 and calcium hydroxide. 

Write an equation for this reaction.

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3a2 marks

A sample of hydrated copper sulphate, CuSO4.xH2O, has a relative formula mass of 285.5. 

Calculate a value for ‘x’. Give your answer to 2 decimal places.

3b5 marks

A student completed an experiment to determine the value of ‘x’ in a hydrated salt. They heated a solid sample of the salt, CuSO4.xH2O, in a crucible for 60 seconds and recorded the following set of results.

Table 1

Mass of empty crucible (g)

29.17

Mass of crucible and solid before heating (g)

32.67

Mass of crucible and solid after heating (g)

32.02

Use the data obtained by the student in Table 1 to calculate a value for ‘x’. Give your answer to 2 decimal places.

3c3 marks

The actual value for x in CuSO4.xH2O is 5.

i)
Suggest a reason for the discrepancy between your calculated value and the actual value.

ii)
Suggest how you could improve the experiment the student did, using the same equipment, to achieve a value closer to the correct value for x. (If you did not achieve a value for x, assume that x = 3.50. This is not the correct answer.)
3d2 marks

Copper (II) sulfate undergoes a displacement reaction with magnesium, because magnesium is a more reactive metal than copper.

i)
Write a balanced equation with state symbols for this reaction.

ii)
Calculate the atom economy of the magnesium product which is formed. 

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4a2 marks

Strong heating of calcium nitrate will cause a thermal decomposition reaction to occur. During the reaction, calcium oxide, nitrogen dioxide and another gas are formed.

Write a balanced chemical equation, with state symbols, for this reaction. 

4b2 marks

A sample of 0.655 g of calcium nitrate was thermally decomposed. 


Calculate the amount, in moles, of the calcium oxide which was produced.
Give your answer to the appropriate number of significant figures.

4c1 mark

Calculate the amount, in moles, of gas which was produced during the thermal decomposition of the 0.655 g sample of calcium nitrate.

Give your answer to the appropriate number of significant figures.

4d5 marks

In a different experiment, a 1.23 g sample of pure lead nitrate, Pb(NO3)2, was heated until it had completely decomposed, in the same way as calcium nitrate in part (a).

Calculate the total volume, in cm3, of nitrogen dioxide gas produced at 50.0 degreeC and 101 kPa.

The gas constant, R, = 8.31 J K-1 mol-1.

Record your answer to the appropriate number of significant figures.

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5a4 marks

Calcium nitrate can be produced by the reaction of calcium sulphate with nitric acid as shown in the equation.

CaSO4 + 2 HNO3 → H2SO4 + Ca(NO3)2 

Excess nitric acid was added to a 3.50 g sample of calcium sulphate forming a 50.0 cm3 solution.

Calculate the concentration of calcium nitrate in this solution.

Give your answer to 3 significant figures.

5b1 mark

A student prepared a solution of a nitrate salt by reacting magnesium oxide with a dilute laboratory acid.

Write the equation for this reaction.

5c2 marks

Calculate the mass in g of magnesium oxide which the student would need to use, to produce 250 cm3 of a 1.25 x 10-2 mol dm-3 solution of the nitrate salt in part (b). 

Give your answer to the appropriate number of significant figures. 

5d4 marks

The student then completed a different experiment. They dissolved 3.75 g of calcium chloride in water to form a solution. This was then added to a flask containing an excess of sulfuric acid. The reaction produced a white calcium sulfate precipitate and hydrogen chloride. 

The percentage yield of the reaction was 87.2 %. 

i)
Write an equation for the reaction. 

ii)
Calculate the mass, in g, of the calcium sulfate precipitate which was formed.

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