OCR AS Chemistry

Topic Questions

ASChemistryOCRTopic Questions1. Development of Practical Skills in Chemistry1.1 Physical Chemistry Practicals

1.1 Physical Chemistry Practicals

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1a1 mark

A student planned an experiment to determine the empirical formula of magnesium oxide by heating magnesium and recording the resulting change in mass.

Some of the equipment the student used is shown in the diagram below.

1-1_1a-ocr-a-as--a-level-sq

Suggest another piece of equipment that would be essential to find the empirical formula of magnesium oxide.

How did you do?

1b2 marks

The method the student used is shown below.

Step 1 Measure the mass of the crucible together with its lid
Step 2 Coil a 10 cm strip of magnesium ribbon and place into the crucible
Step 3 Reweigh the crucible and lid
Step 4 Heat the crucible strongly using the apparatus in the diagram in part (a)
Step 5 Use the tongs to lift the crucible lid slightly during heating
Step 6 As it nears completion, place the crucible on a heat-proof mat and heat strongly for a further 5 minutes
Step 7 Allow the crucible to cool and reweigh the crucible and lid

i)
What is the purpose of Step 5?

ii)
How could you be sure that the reaction had gone to completion?

How did you do?

1c
Sme Calculator3 marks

The student obtained the following results.

Mass of crucible and lid / g 25.68
Mass of magnesium, crucible and lid / g  27.01
Mass of magnesium oxide, crucible and lid / g  27.81

Using the results in the table above, deduce the empirical formula of magnesium oxide.

How did you do?

1d2 marks

Another student performed the same experiment but tapped the residue with tongs to break it up during Step 6.

i)
Suggest a reason for this.

ii)
Suggest how this would affect their calculation of empirical formula.

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2a3 marks

Acid A is a monobasic acid.

A student carried out a titration to find the molar mass of acid A. 

They prepared two solutions:

  • a standard solution of 0.100 mol dm-3 NaOH
  • a 250 cm3 solution prepared from 2.00 g of acid A

The student titrates 25.0 cm3 of the solution of acid A with 0.100 mol dm-3 NaOH in the burette.

They carried out a trial titration, followed by three further titrations.

The diagrams below show the initial and final burette readings for each of the three further titrations.

1-1_q2a-ocr-a-as--a-level-easy-sq

Use the diagrams to complete the burette readings of the further three experiments and calculate the titre for each titration.

Calculate the mean titre that the student should use to analyse the results.

Titration Trial Titration 1 Titration 2 Titration 3
Final burette reading / cm3 21.75      
Initial burette reading / cm3 0.05      
Titre / cm3 21.70      

How did you do?

2b
Sme Calculator1 mark

The uncertainty of each burette reading the student took was ±0.05 cm3.

Calculate the percentage uncertainty in the titre obtained in titration 3.

Give your answer to 2 decimal places.

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2c3 marks

When the student made the standard solution of 0.100 mol dm-3 sodium hydroxide solution, they filled the volumetric flask up to the top of the flask, not the graduation line.

What effect would this have on the mean titre?

Explain your answer.

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2d2 marks

The student washed the conical flask containing the solution of acid A in between each titration, leaving some residual water in the flask.

State whether this would affect the results.

Explain your answer.

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3a1 mark

This question is about finding the enthalpy change of neutralisation of aqueous hydrochloric acid and aqueous sodium hydroxide.

A student reacted 25.0 cm3 of 1.0 mol dm-3 aqueous hydrochloric acid and 25.0 cm3 of 1.0 mol dm-3 aqueous sodium hydroxide and monitored the temperature change using the apparatus shown in the diagram.

calorimetry-hcl--naoh

What piece of apparatus would be most suitable to measure 25.0 cm3 of the two solutions accurately?

How did you do?

3b
Sme Calculator4 marks

The student put the aqueous hydrochloric acid into the polystyrene cup and took the temperature every minute for three minutes. On the fourth minute, they added the aqueous sodium hydroxide, stirred the mixture and continued to take the temperature every minute from the fifth minute.

The student plotted their results shown in the graph below.

enthalpy-change-neut-graph

i)
The student concluded that the maximum temperature change was 5.1 oC. Why does this value not give an accurate value of the expected maximum temperature increase?

ii)
Draw two straight lines of best fit and extrapolate the lines as necessary to find the corrected temperature change at 4 minutes, when aqueous sodium hydroxide was added to aqueous hydrochloric acid.

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3c
Sme Calculator5 marks

Calculate the enthalpy of neutralisation of hydrochloric acid and sodium hydroxide. Use your answer from part (b).
 
The specific heat capacity of the mixture is 4.18 J g-1 K-1. Assume the density of the mixture is 1.00 g cm-3.

Give your answer in kJ mol-1.

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3d3 marks

The literature value of the enthalpy change of neutralisation for the reaction investigated is -57.0 kJ mol-1.

Suggest one improvement to the apparatus and explain why this would improve the accuracy of the results. 

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4a1 mark

When copper oxides in air, it forms copper(II) carbonate basic, CuCO3·Cu(OH)2, which is the green colour that can be seen on copper roofs and statues. The percentage of CuCO3 in CuCO3·Cu(OH)2  can be found by reacting CuCO3·Cu(OH)2 with an excess of sulfuric acid, collecting and calculating the amount of carbon dioxide produced.

The apparatus below can be used to collect the carbon dioxide produced.

1-1_q4a-ocr-a-as--a-level-easy-sq

Suggest what other piece of apparatus could be used to collect gas over water that would improve the accuracy of the volume of gas measured.

How did you do?

4b2 marks

There are other ways of measuring the volume of gas produced, other than collecting gas over water as shown in part (a).

Draw a labelled diagram showing the apparatus used to collect gas by another method.

How did you do?

4c2 marks

Carbon dioxide is slightly soluble in water.

Explain how the method to collect gas using the apparatus shown in part (a) will affect the results.

How did you do?

4d2 marks

There are uncertainties in the measurements taken.

i)
The measuring cylinder shown in part (a) has graduations 2 cm3 apart. The volume of gas collect was 156 cm3.

Calculate the percentage uncertainty when measuring the volume of gas collected.

ii)
The percentage uncertainty associated with measuring the volume of sulfuric acid used is not calculated. Suggest why.

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5a3 marks

Hess' law can be used to determine enthalpy changes for reactions that cannot be determined experimentally.

A student investigated the temperature change in the reactions of potassium carbonate, K2CO3, and potassium hydrogencarbonate, KHCO3, with hydrochloric acid and used their results to determine the enthalpy change of the thermal decomposition of potassium hydrogencarbonate.

They used the method below.

Step 1 Measure 30 cm3 of 1.0 mol dm-3 hydrochloric acid into a polystyrene cup
Step 2 Weigh a weighing boat containing 2.5 g K2CO3
Step 3 Add the K2CO3 to the acid
Step 4 Stir the mixture, record the highest or lowest temperature reached
Step 5 Reweigh the weighing boat
Step 6 Repeat using 3.2 g KHCO3

i)
The student missed a step in the method. What other step did the student need to complete to be able to use their results to calculate the enthalpy change of the reactions?

ii)
What is the purpose of step 5?

How did you do?

5b2 marks

During the reactions between both K2CO3 and KHCO3 with hydrochloric acid, carbon dioxide gas is produced causing fizzing in the reaction mixture.

i)
How could this be a source of error in the method?

ii)
What effect would this have on the calculated energy change q of the reaction?

How did you do?

5c2 marks

When calculating the enthalpy change of reaction, the equation q = mcΔT is used.

State two assumptions made when using this equation.

How did you do?

5d2 marks

The literature value of the thermal decomposition of KHCO3 is +95 kJ mol-1.The student determined the enthalpy change of this reaction to be +88.5 kJ mol-1. 

Why is there a difference between the literature value and the experimental value?

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1a
Sme Calculator3 marks

When copper oxides in air, it forms copper(II) carbonate basic, CuCO3·Cu(OH)2 (s), which is a dull green colour. The percentage by mass of CuCO3 in a sample of copper(II) carbonate basic can be found by reacting copper(II) carbonate basic with acid and measuring the volume of carbon dioxide evolved.

1-1_q1a-ocr-a-as--a-level-hard-sq

A student put exactly 0.50 g of copper(II) carbonate basic to the flask and added 50.0 cm3 of 1.00 mol dm-3 sulfuric acid, which was in excess in the flask. They replaced the bung and collected the gas using the apparatus shown above.

They collected 48.0 cm3 of carbon dioxide gas in the 250 cm3 measuring cylinder, which had graduations every 2 cm3.

Another student repeated the experiment but used an inverted burette graduated to 0.1 cm3 instead of the measuring cylinder.

Which method of gas collection has the lower percentage uncertainty associated with it?

Assume the same volume of gas is collected using the inverted burette.

How did you do?

1b2 marks

Without changing the apparatus used, explain how the student could reduce the percentage uncertainty in using the measuring cylinder to measure the volume of gas collected.

How did you do?

1c3 marks

The student realises that the percentage by mass of copper(II) carbonate in copper(II) carbonate basic should be given to two significant figures.

 
i)
Why should the results be given to two significant figures?

ii)
Suggest how the student might vary the method, without altering the apparatus, which would give a result that could be accurately reported to three significant figures. Explain your answer.

How did you do?

1d5 marks

Suggest two errors in the method that could cause the calculated value for percentage by mass of copper(II) carbonate in copper(II) carbonate basic to be inaccurate. 

For each error, state and explain the effect on the calculated value of percentage by mass and how the method could be altered to reduce the inaccuracy in the value.

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2a6 marks

A student performed a titration to identify an unknown Group 1 metal carbonate.

The student prepared 250 cm3 of a standard solution using 3.00 g of the unknown metal carbonate. They then used 25.0 cm3 of this solution and titrated it with 0.250 mol dm-3 of hydrochloric acid.

Describe how the student prepared the standard solution of the unknown metal carbonate.

How did you do?

2b1 mark

The student records their measurements and chemicals used. They write the concentration of hydrochloric acid used as 0.25 mol dm-3 instead of 0.250 mol dm-3.

Explain why the two concentrations do not mean the same.

How did you do?

2c
Sme Calculator3 marks

The student's results are shown in the table below.

  Trial Titration 1 Titration 2 Titration 3
Final burette reading / cm3 18.15 35.55 17.6 35.02
Initial burette reading / cm3 0.05 18.15 0.15 17.45
Titre / cm3 18.10 17.4 17.45 17.57
Mean titre / cm3 17.47

Identify any errors in their results table.

How did you do?

2d5 marks

The student used the following steps to perform the titration.

Step 1 Measure 25.0 cm3 of the unknown metal carbonate solution using a volumetric pipette and transfer into a conical flask
Step 2 Add 3 to 4 drops of methyl orange
Step 3 Place the conical flask onto a white tile
Step 4 Rinse the burette with distilled water
Step 5 Using a funnel, fill the burette with 0.250 mol dm-3 hydrochloric acid
Step 6 Titrate the metal carbonate solution with the hydrochloric acid, constantly swirl until the solution turns from yellow to orange - this is the rough titration
Step 7 Repeat the titration until two results are concordant, but add the hydrochloric acid drop-by-drop as the end point is approached

Identify any errors in the method.

State and explain their effect on the unknown metal's calculated relative atomic mass.

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3a2 marks

The enthalpy change for the following reaction is known as the enthalpy change of hydration of anhydrous copper(II) sulfate. It is an exothermic reaction.

CuSO4 (s) + 5H2O (l) → CuSO4⋅5H2O (s)

The enthalpy change cannot be measured directly. Suggest why not.

How did you do?

3b1 mark

The enthalpy change of the hydration of copper(II) sulfate can be determined experimentally using Hess' law. This involves recording the temperature change that occurs when both anhydrous copper(II) sulfate and hydrated copper(II) sulfate are dissolved in an excess of water.

The equations for the reactions are shown below, where nH2O represents an excess of water.

CuSO4(s) + nH2O → CuSO4(aq)

CuSO4.5H2O(s) + nH2O →  CuSO4(aq)


Explain why an excess of water is used.

How did you do?

3c3 marks

The method below is used to determine the temperature change when anhydrous and hydrated copper(II) sulfate is added to water. The measured temperatures are then used to determine the enthalpy change of the reactions.

Step 1 Measure 50 cm3 of water and add to a polystyrene cup placed inside a 250 cm3 glass beaker
Step 2 Accurately weigh a weighing bottle containing approximately 5.0 g of anhydrous copper(II) sulfate
Step 3 Measure and record the temperature of the water
Step 4 Add the anhydrous copper(II) sulfate to the water, and continually stir. Measure and record the highest or lowest temperature 
Step 5 Reweigh the weighing bottle
Step 6 Repeat steps 1 to 5 using 7.8 g hydrated copper(II) sulfate

A student suggests that the experiment should be altered to record the mass of the empty polystyrene cup before adding the water and then again at the end of the reaction.

Discuss if this alteration could have any effect on the calculated enthalpy changes.

How did you do?

3d
Sme Calculator1 mark

The reaction scheme for the formation of hydrated copper sulfate from its anhydrous form is shown below.

1-1_q3d-ocr-a-as--a-level-hard-sq

Use the reaction scheme, Hess' law and the data provided in the table below to calculate the enthalpy of hydration for anhydrous copper(II) sulfate.

  ΔH / kJ mol-1
Enthalpy change, ΔH1 -66.1
Enthalpy change, ΔH2 + 11.0

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4a2 marks

A student investigated the enthalpy change of combustion of different liquid fuels using a spirit burner and the apparatus shown below.

1-1_q4ah-ocr-a-as--a-level-hard-sq

They used the following method.

Step 1 Weigh the spirit burner and methanol
Step 2 Measure 150 cm3 of water and pour into the beaker
Step 3 Record the initial temperature of the water
Step 4 Place the spirit burner under the beaker and light
Step 5 Use the thermometer to stir the water
Step 6 After about 5 minutes, extinguish the flame and record the maximum temperature reached by the water
Step 7 Reweigh the spirit burner and methanol

The student repeated the method using ethanol and propanol.

When analysing the results, the student noticed that when the mass of the three fuels burnt was the same, the resulting temperature increase was different.

Suggest why.

How did you do?

4b1 mark

The measured temperature increase is used to calculate the energy transferred in the experiment. The student thought that if they used the same initial temperature and final temperature in each experiment, the energy transferred in each experiment would be the same.

State the assumption that the student must make for this to be the case.

How did you do?

4c3 marks

After extinguishing the flame after the final temperature was recorded, the student tidied their equipment away before reweighing the spirit burner.

State and explain the effect, if any, on the calculated value of the enthalpy change of combustion.

How did you do?

4d3 marks

When performing the experiment using another fuel, the student accidentally spilt some of the water when transferring it to the beaker.

State and explain the effect, if any, on the calculated value of the enthalpy change of combustion.

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5a6 marks

This question is about hydrated aluminium sulfate, Al2(SO4)3⋅xH2O (s).

Describe how the amount of water of crystallisation, x, in the formula of hydrated aluminium sulfate can be determined experimentally. 

How did you do?

5b2 marks

In one experiment, the student did not remove all of the water of crystallisation.

Explain how their calculated value of the number of moles of water of crystallisation, x, would differ from the actual value of x.

How did you do?

5c3 marks

In another experiment, the student heated the hydrated aluminium sulfate very strongly for 30 minutes.

i)
Give a reason why this may cause their calculated value of x to be incorrect.

ii)
State if their calculated value of x would be greater or smaller than the actual value of x. Explain your answer.

How did you do?

5d3 marks

The student decided to start with a larger mass of hydrated aluminium sulfate as they said that this would reduce the percentage uncertainty in their calculated value of x.

i)
Explain why this would reduce the percentage uncertainty.

ii)
Give a reason why using a larger initial mass of hydrated aluminium sulfate may produce anomalous results.

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1a4 marks

A student was asked to plan and carry out a practical to determine the Mr of magnesium by reacting it with dilute hydrochloric acid at room temperature and pressure.

Plan how the student could determine the Mr of magnesium.

In your answer include detail of the practical procedure carried out and measurements that would need to be taken.

How did you do?

1b2 marks

Write a balanced symbol equation for this reaction.

How did you do?

1c
Sme Calculator3 marks

The student found that 0.11 g of magnesium ribbon produced 110 cm3 of hydrogen, at room temperature and pressure, when reacted with an excess of 1.0 mol dm-3 hydrochloric acid.

Calculate the Mr of the magnesium. Give your answer to 1 decimal place.

How did you do?

1d2 marks

Another student followed the same procedure to complete the practical using 0.11 g of magnesium powder but only recorded a volume of 92.0 cm3.

Suggest why the student recorded a volume lower than expected. Explain your reasoning.

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2a4 marks

Lactic acid, C3H6O3, is a chemical byproduct of anaerobic respiration.

Bacteria in yoghurt will produce it, as will bacteria in our stomach.

Bacteria in beer will also ferment glucose, producing lactic acid by anaerobic respiration.

The increase in lactic acid decreases the pH of the beer and gives it its characteristic sour taste.

A student devised an experiment to investigate the concentration of lactic acid in a sample of beer.

  1. Transfer 15.0 cm3 of beer to a volumetric flask using a beaker.
  2. Make up the volume of the beer to 300 cm3.
  3. Use a pipette to take a portion of 30.0 cm3 and add it to a conical flask.
  4. Give the conical flask an initial swirl and then place on the bench.
  5. Add a few drops of phenolphthalein indicator to the lactic acid in the conical flask.
  6. Titrate samples of 30.0 cm3 of this solution with a 0.0750 mol dm-3 solution of sodium hydroxide solution.

Complete the following table stating how an improvement to the procedure could be achieved:

Improvement

Method to achieve improvement

Reduce the overall measurement uncertainty in the titration

  

Improve the chance of obtaining concordant results

  

Improve accuracy of titrations by rinsing the beaker with...

  

Rinse burette with … for the final rinse

  

How did you do?

2b
Sme Calculator3 marks

The student's incomplete results table is shown below:

Titration

Rough

1

2

3

Final reading (cm3)

23.20

24.10

  

52.20

Start reading (cm3)

  

0.15

0.05

25.70

Titre (cm3)

23.10

  

23.90

  

Complete:   

i)
Complete the student's results table.

ii)
Calculate the mean titre to 2 decimal places.

How did you do?

2c
Sme Calculator2 marks

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

Suggest the maximum percentage uncertainty in using this piece of equipment using your mean titre calculated in part (b).

How did you do?

2d6 marks

Another student is required to make up 300 cm3 of an aqueous solution that contains a known mass of lactic acid. The student is provided with a sample bottle containing the lactic acid.

Describe the method, including full apparatus and practical details, that the student should use to prepare this solution.

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3a4 marks

The enthalpy of combustion of an unknown liquid fuel can be determined by carrying out a calorimetry experiment.

i)
Draw a labelled diagram of the apparatus set-up required to carry out such an experiment.

ii)
State one way to reduce uncertainties in this experiment.

How did you do?

3b2 marks

A student used the following procedure to carry out a calorimetry experiment.

Procedure

  1. Measure 100 cm3 of water into a calorimeter using a measuring cylinder.
  2. Record the initial temperature of the water using a thermometer.
  3. Heat the water using the flame from the burning fuel and record the final temperature.
  4. Measure the final mass of the spirit burner with the liquid fuel.

Which crucial step in the calorimetry experiment has been left out in the method above? Explain your reasoning.

How did you do?

3c
Sme Calculator4 marks

The student obtained the following results:

    • Mass of water in calorimeter = 350 g
    • Initial temperature of water = 9 oC
    • Final temperature of water = 36 oC
    • Mass of liquid fuel burned = 3.15 g
i)
Calculate the amount of energy required to raise the temperature of the water from its initial to its final temperature. 
ii)
Calculate the amount of liquid fuel burned in moles (Mr of the liquid fuel is 63.2 g mol-1).
iii)
Calculate the enthalpy of combustion of 1.0 mole of the liquid fuel. State the correct units.

How did you do?

3d2 marks

The actual value of ΔHc of the unknown liquid is greater than the experimental value calculated above.

Give two reasons for this.

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4a2 marks

A student wanted to determine the formula of hydrated magnesium sulfate, MgSO4•xH2O.

The student used the following procedure to determine the water of crystallisation in the magnesium sulfate salt. 

Procedure

  1. Using a 2 d.p. balance, record the mass of an empty crucible.
  2. Add a small amount of hydrated magnesium sulfate to the crucible, re-weigh and record the mass.
  3. Gently heat the crucible for 2 minutes.
  4. Allow the crucible to cool, re-weigh and record the mass.

State two ways that the uncertainty in measuring mass can be reduced.

How did you do?

4b2 marks

Explain how heating until a constant mass is reached will improve the results of the student's experiment.

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4c
Sme Calculator2 marks

Explain why the following masses of magnesium sulfate would give inaccurate results.

i)
50.0 g of magnesium sulfate

ii)
0.01 g of magensium sulfate

How did you do?

4d
Sme Calculator4 marks

The student's results are shown in the table below.

 

Mass / g
Empty crucible 3.89
Crucible + hydrated magnesium sulfate 7.10
Crucible + anhydrous magnesium sulfate 5.46

Determine the formula of the hydrated magnesium sulfate.

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