Amount of Substance (OCR A Level Chemistry)

Exam Questions

3 hours49 questions
1a3 marks

This question is about the use of different formulae to represent chemicals.

Explain what is meant by the terms empirical formula and molecular formula, using a named alkene as an example.

1b
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4 marks

Analysis of an unknown hydrocarbon, X, showed the following percentage composition by mass: C = 82.8, H = 17.2 

What is the empirical formula of the unknown hydrocarbon? 

(Ar: C = 12.0 , H = 1.0) 

1c2 marks

A student suggests that the structure of the unknown hydrocarbon, X, could be: 

cyclobutane-skeletal-formula

Explain, with reasons, whether the student is correct.

1d
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3 marks

Unknown hydrocarbon, Y, is analysed and a student completes the following calculation to determine that the compound is ethane: 

  C H
Value 81.8% 18.2%
Atomic mass 12.0 1.0
Moles 6.817 18.2
Ratio 1 2.67 → 3
Empirical formula CH3  
Compound Y C2H6  
 

Identify the error that the student has made and explain how they should have completed the calculation.

1e1 mark

What extra piece of information could have told the student that their answer to part (d) was incorrect?

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2a1 mark

This question is about anhydrous and hydrated salts.

Explain the difference between an anydrous salt and a hydrated salt.

2b1 mark

Potassium carbonate can have the formula K2CO3xH2O.

Calculate the relative formula mass of K2CO3. 

(Ar: K = 39.1, C = 12.0, O = 16.0)

2c
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2 marks

A 10.0 g sample of K2CO3•xH2O contains 11.5% by mass of water. 

Calculate the mass of K2CO3 and H2O in the sample.

2d
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3 marks

Complete the following table to calculate the water of crystallisation present in the K2CO3•xH2O.

 
  K2CO3 H2O
Mass (g)    
Mr    
Moles    
Ratio    

 

x = _____    

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3a2 marks

A student carries out an investigation to identify the carbonate of an unknown metal, X2CO3

They measure the mass loss when the X2CO3 reacts with excess hydrochloric acid to form the metal chloride XCl. 

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

 
X2CO3 (___) + 2HCl (___) → 2XCl (aq) + _____ (g) + _____ (l)
3b
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2 marks

The student carries out the reaction using this method:

 
Step 1   Add 50 cm3 HCl (aq) to a conical flask and weigh.
 
Step 2   Add X2CO3 to the conical flask and immediately reweigh.
 
Step 3   After 5 minutes, reweigh the conical flask and contents.
 
Results
 
Mass of conical flask + HCl (aq)  142.96 g
Mass of conical flask + X2CO3 + HCl (aq) before reaction  163.32 g
Mass of conical flask + contents after 5 minutes  160.57 g

 
Calculate the amount, in mol, of CO2 released in the reaction.

 
Amount of CO2 = __________ mol
3c2 marks

The reactant X2CO3 forms the product XCl. 

Explain, with reasons, which group of the periodic table the metal X belongs to.

3d1 mark

Calculate the mass of the unknown metal carbonate, X2CO3, that the student used in their experiment.

3e
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3 marks

The student calculates the molar mass of the unknown metal carbonate, X2CO3, to be 325.76 g mol-1

Identify metal X by calculating the relative atomic mass.

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4a
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2 marks

This question is about the reactions of lithium.

Lithium reacts with fluorine to form lithium fluoride. 

2Li (s) + F2 (g) → 2LiF (s)
 

Calculate the percentage atom economy for this reaction.

(Ar data:  Li = 6.9, F = 19.0)

4b
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3 marks

Calculate the maximum mass of lithium fluoride that can be produced from 0.345 g of lithium.

4c
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2 marks

At room temperature and pressure, one mole of gas has a volume of 24.0 dm3

Calculate the volume of fluorine required to react with 0.345 g of lithium.

4d
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3 marks

At room temperature, calculate the pressure exerted on the flask by the volume of fluorine calculated in part (c). 

Give your answer in kPa. 

(Gas constant, R = 8.314 J mol−1 K−1)

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5a
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1 mark

This question is about the preparation of magnesium sulfate.

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 why reaction 3 has the lowest atom economy.

5b
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3 marks

One mole of any chemical contains 6.02 x 1023 particles. 

A student uses 2.43 g of magnesium to produce magnesium sulfate, using reaction 1. 

Mg + H2SO4 → MgSO4 + H2 
 

Calculate the number of atoms of hydrogen that are produced.

 
(Ar: Mg = 24.3)
5c
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3 marks

Another student prepares magnesium sulfate using 2.015 g of magnesium oxide. 

MgO + H2SO4 → MgSO4 + H2
 

The student produces 3.010 g of magnesium sulfate. 

Calculate the percentage yield for the student's reaction. 

(Mr: MgO = 40.3, MgSO4 = 120.4)
5d
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2 marks

A third student uses magnesium carbonate to produce magnesium sulfate. 

MgCO3 (s) + H2SO4 (aq) → MgSO4 (aq) + CO2 (g) + H2O (l)
 

The student adds universal indicator solution to the sulfuric acid to monitor the reaction. 

State two observations that the student would expect to see during their reaction.

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1a
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3 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.

1b
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1 mark

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

Write a balanced symbol equation for this reaction.

1c
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3 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 3 significant figures.

1d
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4 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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2a
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5 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 oC. 

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

2b
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4 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.

2c
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3 marks

The Mr of chrysotile is 520.2. 

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

2d
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2 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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3a
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2 marks

A sample of hydrated copper sulphate, CuSO4.xH2O, has a relative formula mass of 285.6. 

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

3b
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5 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.

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

3c
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2 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)

Explain how you could improve the experiment the student did, using the same equipment, to achieve a value closer to the correct value for x.

3d
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2 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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4a
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2 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

4b
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2 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.

4c
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1 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.

4d
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5 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 oC and 101 kPa.

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

Record your answer to the appropriate number of significant figures.

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5a
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3 marks

When magnesium reacts with hydrochloric acid, the following reaction occurs:

Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)  

During the reaction, the hydrogen produced occupies 103 cm3 at 25.0 oC and 100 kPa. 

Calculate the amount, in moles, of hydrogen gas produced during the reaction. 

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

5b
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2 marks

Using your answer to part (a), calculate the exact volume in cm3 of 0.15 mol dm-3 HCl, which would be needed to produce that amount of hydrogen gas. Give your answer to 1 decimal place.
 

5c
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2 marks

A student completed the same reaction as in part (a), using 3.75 g of magnesium.

Calculate the mass of magnesium chloride produced by the student during this reaction.  

5d
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2 marks

Calculate the number of atoms of hydrogen produced during the students reaction in part (c).

The Avogadro constant, L = 6.022 x 1023.

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1a
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4 marks

Magnesium carbonate is a common antacid that is used to react with and neutralise excess stomach acid, HCl, with the following equation:

MgCO3 (s) + 2HCl (aq) rightwards arrow MgCl2 (aq) + CO2 (g) + H2O (l)

The total mass of the reactants is 9.91 g.

What volume of CO2 is produced at RTP?

1b
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3 marks

Another magnesium salt, magnesium sulfate, can exist in hydrated or anhydrous forms. 

6.31 g of a sample of magnesium sulphate, MgSO4.xH2O, was strongly heated until no further change in mass was recorded. 

Calculate the number of moles of water of crystallisation in the magnesium sulphate sample given that 3.61 g of solid remained.

1c
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3 marks

Another sample of hydrated magnesium sulphate, MgSO4.xH2O, is found to contain 51.1% water by mass.

Calculate the value of x in this hydrated salt.

1d2 marks

Describe the effect on the value of x obtained if the salt was not heated strongly to a constant mass.

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2a
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3 marks

Carvone is an organic compound containing carbon hydrogen and oxygen.

Complete combustion of 0.1526 g carvone produces 0.4470 g of carbon dioxide and 0.1281 g of water.

Determine the empirical formula of carvone, showing your working.

2b
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3 marks

0.146 g sample of carvone, when vaporised, had a volume of 0.0341 dm3 at 150 oC and 100.2 kPa.
Calculate its molar mass showing your working.

2c
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2 marks

Using your answer to part b), determine the number of molecules of carvone in 0.146 g.

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3a
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4 marks

Fireworks contain elements to give them their colour, as well as oxygen-containing compounds to facilitate speedy combustion.

In a firework, solid potassium nitrate, KNO3, decomposes to form solid potassium nitrite, KNO2, and oxygen, O2

Calculate the mass, in g, of potassium nitrate, KNO3, required to make 1.5 g of oxygen.

Give your answer to 2 significant figures. 

3b
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1 mark

Calculate the volume of gas at RTP, in dm3, that is produced in the reaction outlined in part (a).
Give your answer to 2 significant figures. 

3c
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4 marks

Potassium can form a superoxide, KO2 (s), which will react with carbon dioxide, CO2 (g), to produce potassium carbonate, K2CO3 (s) and oxygen, O2 (g).

Calculate the volume, in dm3, of carbon dioxide which will react with 5.00 g of the superoxide.

Give your answer to 3 significant figures. 

3d3 marks

During the reaction outlined in part (c), 2.61 g of potassium carbonate, K2CO3, was produced when the experiment was carried out.

Calculate the percentage yield for the production of potassium carbonate. Give your answer to 2 decimal places. 

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4a
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4 marks

Salts are commonly made during neutralisation reactions.

3.75 g of zinc oxide, ZnO (s), was added to 75.0 cmof 1.00 mol dm-3 of hydrochloric acid (aq) producing a salt. 

Calculate the limiting reagent in the reaction. Give your answer to 2 significant figures. 

4b
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2 marks

During the reaction, 4.35 g of the salt was produced.

Calculate the percentage yield of the reaction. Give your answer to 3 significant figures.

4c
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3 marks

Calculate the atom economy of the reaction, why a high atom economy is beneficial and what could be done to improve the atom economy to this reaction.

4d2 marks

Evaluate which of ZnO and HCl should ideally be in excess when the aim is to produce a pure, dry salt and explain your answer.

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5a
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4 marks

Ammonia can be oxidised by bacteria to N2O, or by an inorganic catalyst to NO.

90 cm3 ammonia gas, NH(g), is combusted in oxygen, O(g) in a closed system to produce nitrogen monoxide and water, H2O (l).

What is the total volume of gas remaining in the system when 90 cmof ammonia is combusted completely with 50 cm3 of oxygen?

5b1 mark

The temperature of the reaction is kept constant.

Sketch a line on the graph below that shows the correct relationship between pressure and begin mathsize 16px style 1 over volume end style.

Pressure vs 1/volume IB Chem SQ 1.2

5c
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3 marks

At 25 oC and 100 kPa, a gas occupies a volume of 35 dm3.

Calculate the new temperature, in oC, of the gas if the volume is decreased to 15 dm3 at constant pressure. 

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