Further Organic & Inorganic Chemistry Practicals (OCR A Level Chemistry)

Exam Questions

3 hours30 questions
1a1 mark

A student was required to purify an impure sample of benzoic acid.

State how the student can purify the impure sample.

1b4 marks

Describe how the student would purify the sample of impure benzoic acid

1c3 marks

The student tested the melting point of the pure benzoic acid.

When preparing for and carrying out a melting point test, identify three key skills that must be adhered to.

1d2 marks

Explain what the student would expect to see if the product was pure when the melting point was tested.

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2a1 mark

A student prepared a sample of aspirin from salicylic acid and ethanoic anhydride.

Other than cost, give one advantage of using ethanoic anhydride instead of ethanoyl chloride. 

2b1 mark

Once the student has produced the aspirin, they need to recrystallise the product. 

Explain why this is an important step.

2c1 mark

Suggest a suitable solvent to use in the recrystallisation of aspirin.

2d3 marks

Having cooled and recrystallised the aspirin, state the final three steps in the purification of the solid that the student should take.

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3a4 marks

A student carried out a practical test to determine the identity of four unlabelled bottles containing separate solutions of propene, propan-1-ol, propanal and propanone. The results are shown in Table 1

1-3_q3a-ocr-a-as--a-level-easy-sq

Use Table 1 to identify solutions A-D

3b1 mark

Write an ionic equation to represent the reaction occurring when a silver mirror is formed from the [Ag(NH3)2]+ ion in Tollens’ reagent. 

3c4 marks

A further unlabelled solution contained an unknown potassium halide solution. Explain how a student could form a precipitate of the unknown halide ion so it can be identified. 

3d2 marks

The student identifies the unknown halide solution as potassium bromide.

Write the ionic equation for the formation of precipitate. Include state symbols in your answer.

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4a4 marks

This question is about electrochemical cells.

In order to measure the electromotive force (EMF) of a cell made from copper and zinc half cells, the following apparatus shown in Figure 1 needs to be set up.

Figure 1
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Label parts A and B and suggest suitable solutions for C and D. 

4b1 mark

Describe what should be done to the metal rods before they are connected to the cell.

4c1 mark

A student set up the apparatus shown in Figure 1 in part a). Their EMF reading was negative. Suggest what the student should do to get a positive EMF. 

4d1 mark

State the concentrations required for solutions C and D.

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5a2 marks
A student prepared methyl 3-nitrobenzoate according to the following procedure

:

Step 1: Add 2.0 g of methyl benzoate to a small conical flask followed by 4 cm3 of concentrated sulfuric acid

Step 2: A nitrating mixture of 1.5 cm3 concentrated sulfuric acid and 1.5 cm3 concentrated nitric acid is made 

Step 3: The nitrating mixture is added slowly over 15 minutes to the conical flask 

Step 4: Once the product has solidified, the contents of the flask are poured over crushed ice 

Step 5: Once the ice has melted, the product can be filtered and removed

State the equation for this reaction.

State symbols are not required.

5b3 marks

Suggest one improvement each to Step 1, 2 and 3.

5c1 mark

Name a suitable piece of apparatus that is required for Step 5 in part a).

5d2 marks

To purify the product, the student recrystallised the methyl 3-nitrobenzoate and heated the solution using a hot plate. The solution used contained water and ethanol. 

Explain why this is an important safety precaution to follow.  

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1a3 marks

A student has two unlabelled samples of colourless liquid. The samples are known to be cyclohexanol, C6H11OH, and t-butanol, (CH3)3COH.

Describe a chemical test to distinguish between cyclohexanol and t-butanol. Your answer should explain the expected observations.

1b3 marks

Cyclohexanol and t-butanol both undergo dehydration reactions by heating in the presence of sulfuric acid.

Identify the carbocations formed during the dehydration reactions.

Use the carbocations to explain why t-butanol requires a lower temperature for the reaction to occur.

1c3 marks

A student has correctly identified the cyclohexanol sample.

They set up the following equipment to determine the boiling point of the cyclohexanol.

Distillation apparatus

Explain three ways that the student could improve their equipment.

1d2 marks

The purity of a liquid can be determined by measuring its boiling point. An alternative method to distillation would be to boil a tube of the sample in an oil bath.

The expected boiling point of a cyclohexanol sample is 161 °C but following distillation it was found to be 168 °C.

Suggest why the boiling point was higher than expected and why measuring the boiling point is not the most accurate method to confirm the identity of a known sample.

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2a3 marks

In a multi-step reaction, benzoic acid can be prepared by hydrolysing ethyl benzoate using sodium hydroxide.

Write an equation for this reaction.

2b5 marks

A student used the following method to make benzoic acid.

    1. Place 3.0 cm3 of ethyl benzoate (density = 1.05 g cm−3, Mr = 150.0 g mol-1) in a pear-shaped flask
    2. Add 25 cm3 (an excess) of sodium hydroxide solution.
    3. Set the pear-shaped flask up for reflux.
    4. Heat the mixture, under reflux, for approximately 30 minutes.
    5. Leave the mixture to cool to room temperature.
    6. Add 50.0 cm3 of 2 mol dm−3 hydrochloric acid
    7. Collect the crude product using suction filtration.

i)
This reaction will not go to completion unless sodium hydroxide solution is added. Prove by calculation that this reaction went to completion and explain why excess sodium hydroxide is necessary.

ii)
Suggest an essential step which is missing from the method above, and why this step is necessary. 
2c3 marks

The crude product mixture from part b will be purified by recrystallisation. Describe how to identify a suitable solvent for the recrystallisation process.

2d4 marks

After successful recrystallisation, the melting point of the benzoic acid was determined to assess the purity of the sample. The expected melting point was 122.3 °C.

i)
Describe how you could measure the melting point of benzoic acid.

ii)
State two observations to suggest the benzoic acid sample is impure.

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3a5 marks

A standard hydrogen electrode can be used to determine the standard electrode potential of a Cu (s) / Cu2+ (aq) electrode.

Draw a labelled diagram of the apparatus which would be used to determine the standard electrode potential of Cu (s) / Cu(aq). 

State the necessary conditions for this cell, which would allow the standard electrode potential to be measured.

3b5 marks

The table shows the half equations and standard electrode potentials of some half cells. 

Electrode half-equation Electrode potential / V
Cl2 (g) + 2e → 2Cl (aq) + 1.36
Ag+ (aq) + e → Ag (s)  + 0.80
Fe3+ (aq) + e → Fe2+ (aq) + 0.77
Cu2+ (aq) + 2e → Cu (s) + 0.34
Fe2+ (aq) + 2e → Fe (s) – 0.44

i)
Deduce the equation for the overall cell reaction of a cell that has an e.m.f. of 1.24V. 
ii)
Identify the negative electrode.
iii)
Give the conventional cell representation for this cell.
iv)
State the standard conditions used for electrochemical cells.
3c3 marks

An experiment was carried out to determine the e.m.f of this cell.

Fe (s) |  Fe3+ (aq)  ||  H(aq) |  H2 (g)  |  Pt (s)

i)
The iron used as the electrode is rubbed with sandpaper prior to use. State the reason for this.

ii)
A salt bridge can be prepared by dipping a piece of filter paper into a sodium carbonate solution. Explain why such a salt bridge would not be suitable for use in this cell.
3d8 marks

In a Daniell cell, zinc is a more reactive metal than copper and this type of cell can be used to provide electrical energy. The conventional representation for the Daniell cell is;

Zn (s) | Zn2+ (aq)  ||  Cu2+ (aq) | Cu (s)

i)
State the half-equations for the reactions occurring at the electrodes.

ii)
A Daniell cell was set up using 150 cm3 of a 1.2 mol dm–3 copper(II) sulfate solution. The cell was allowed to produce electricity until the concentration of the copper(II) ions had decreased to 0.60 mol dm–3Calculate the decrease in mass of the zinc electrode. Show your working.

iii)
You are provided with the Daniell cell including a zinc electrode of known mass as referred to in part (ii).Briefly outline how you would carry out an experiment to support your answer to part (ii).

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4a6 marks

A student prepared a sample of aspirin by refluxing salicylic acid with an excess of acetic anhydride in the presence of a phosphoric (V) acid catalyst. The reaction was quenched with water, cooled and crystallised. The crude product was collected by suction filtration, washed and then purified by recrystallisation. Thin-layer chromatography was then used to ascertain whether the product had been sufficiently purified by recrystallisation. 

Outline the method that the student would use to determine whether the product was pure, including the results that the student would expect to see. You should include a labelled diagram to support your answer.

4b3 marks

Outline how the student could identify the positions of any compounds on the TLC plate and describe the expected results to show that the crude product had been purified by recrystallisation.

4c3 marks

There is an extensive range of over the counter and prescription medicines, that contain a variety of chemicals. TLC is often one of the first methods used to analyse medicines.

Give three advantages of using TLC in the analysis of such medicines.

4d
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3 marks

The structure of aspirin is shown below.

1-13

Standard over the counter aspirin tablets are around 85% pure. One of the common impurities found in aspirin is ethanoic acid from the manufacturing process. A student analysed a 325 mg aspirin tablet and found it to contain 1.56 x 10-3 moles of aspirin.

Evaluate the safety of the tablet for human consumption.

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5a1 mark

Aspirin, shown below, can be synthesised by the acylation of 2-hydroxybenzoic acid using either ethanoyl chloride or ethanoic anhydride.

Aspirin

Write an equation to show the formation of aspirin using ethanoic anhydride.

5b2 marks

State two reasons why ethanoic anhydride is used in preference to ethanoyl chloride for the acylation.

5c5 marks

A student uses the following method to make aspirin (Mr 180.158 g mol-1) in the school laboratory.

    1. Place 1.50 g of 2-hydroxybenzoic acid (M = 138.121 g mol-1) and 3.00 cm3 of ethanoic anhydride (M = 102.089 g mol-1) in a 50.0 cm3 pear-shaped flask.
    2. Add 4 drops of phosphoric (V) acid and swirl to mix.
    3. Set the pear-shaped flask up for reflux.
    4. Heat the mixture with a boiling water bath, for approximately 5 minutes.
    5. Carefully, add 2.00 cm3 of water down the reflux condenser, without cooling.
    6. When the reaction is complete, pour the mixture into a 100 cm3 beaker containing 30.0 cm3 of cold water.
    7. Stir the mixture and rub the sides of the beaker with a stirring rod to promote crystallisation.
    8. Stand the mixture in an ice bath to complete crystallisation.
    9. Collect the product using suction filtration.

Calculate the expected mass of aspirin, assuming the student achieves an 85% yield in their crude product. You may assume that the density of ethanoic anhydride is 1.00 g cm-3.

In your answer, you should identify the limiting reagent.

5d5 marks

Describe how the student could purify the aspirin after it has been filtered.

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1a4 marks

This question is about the preparation of aspirin.

The salicylic acid required to prepare aspirin must be pure. Describe how the salicylic acid can be purified and explain why each step is required. 

1b4 marks

A student prepared a sample of aspirin by following the two stages outlined:  

Stage 1
  • Add a known mass of the purified salicylic acid into a dry pear shaped flask
  • Add an excess of 5 cm3 of ethanoic anhydride
  • Add by 8 drops of concentrated phosphoric acid
  • Attach a condenser to the flask
    In a fume cupboard,  warm the
    mixture with swirling until the solid has dissolved
  • Continue to warm for another 5 minutes
 
Stage 2
 
  • Carefully, add 5 cm3 of cold water to the solution
  • Stand the flask in cold water to promote precipitation
  • Filter off the product and wash the product with a little cold water
  • Transfer the product to a watch glass and leave to dry overnight
  • Once dry, weigh the product and record the mass.  

Suggest four improvements to the method the student followed.

1c2 marks

Draw a labelled diagram of the apparatus used to filter the mixture in Stage 2 outlined in part (b).

1d3 marks

The following reaction scheme is for the preparation of aspirin.

 
Production of aspirin
 

The average percentage yield for the industrial synthesis of aspirin is around 79.2%. A student uses 1.05 g of salicylic acid with an excess of ethanoic anhydride for their aspirin synthesis. 

Calculate the minimum mass of aspirin that they would need to produce to meet industrial standards.

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2a6 marks

This question is about the preparation of methyl 3-nitrobenzoate.

Outline a method for the nitration of methyl benzoate. Include safety considerations and equations for the reaction. You do not need to discuss purification of the crude product. 

2b4 marks

Describe the purification process of the product using a suitable solvent.

2c1 mark

A student tested the methyl 3-nitrobenzoate product produced in part (a) and found the melting point to be lower than expected 78 ℃ data book value.

 Explain how the student can obtain a more accurate value.

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3a3 marks

Figure 1 shows a student's diagram of the standard hydrogen electrode.

Figure 1

4-2

Identify the error that the student has made and explain why it is incorrect.

3b2 marks

The approximate cost of platinum in the UK is £22 500 per kg.

Explain how the design of the platinum electrode minimises the cost while maximising its efficiency. 

3c2 marks

A student set up an electrochemical cell as shown in Figure 2.

Figure 2

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The student made the silver electrode by adding 7.78 g of silver sulfate to 25.0 cm3 of water at 298 K. 

The solubility of silver sulfate in water at 298 K is 4.73 g dm-3.

Explain if the setup in Figure 2 can be used to measure the standard electrode potential of the Ag / Ag+ half cell. Show your working.

3d2 marks

The cell represented in Figure 3 was set up under standard conditions.

Figure 3

6

Explain why this electrochemical cell cannot measure the standard electrode potential of the Fe2+ / Fe3+ half cell.

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4a2 marks

Tollens’ reagent or diammine silver hydroxide, [Ag(NH3)2]+OH-, is a mixture of liquid ammonia and silver nitrate. Tollens’ reagent is prepared in the laboratory in two steps. 

The first step is to react silver nitrate and sodium hydroxide to form a brown precipitate of silver(I) oxide. This precipitate is then dissolved in concentrated ammonia to form the reagent. 

Write two equations, including state symbols, to show the laboratory formation of Tollens’ reagent.

4b2 marks

Write an equation, including state symbols and give the observation for the reaction of Tollens’ reagent with benzaldehyde. 

4c2 marks

Suggest why the student should only use clean and dry glassware for this practical. 

4d1 mark

Potassium dichromate is less expensive than Tollens' regent and can also be used to test for aldehydes.

Explain why Tollens' reagent is specifically used. 

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5a1 mark

A mixture of ethyl benzoate 5 cm3 and sodium hydroxide solution 15 cm3 was refluxed in a round bottomed flask fitted with a water condenser on a water bath at a temperature of 90 °C  for 30 minutes. A salt was formed in the reaction.

The solution was then cooled and acidified with hydrochloric acid. The resultant acidified solution was then cooled in an ice bath forming a precipitate of benzoic acid.

State the equation to form the salt. 

5b4 marks

Describe the purification process of the product using a suitable solvent.

5c1 mark

According to a data book, the melting point of benzoic acid is 122.3 °C. 

The product formed in part (a) was found to have a lower melting point than expected.

Explain how the student can obtain a more accurate value.

5d3 marks

The mass of benzoic acid produced in the reaction was 1.2 g and the percentage yield of benzoic acid formed was calculated to be 80.53%.

Calculate the mass of ethyl benzoate that was used in the reaction.

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