Organic Chemistry Practicals (A Level only) (AQA A Level Chemistry)

Exam Questions

3 hours30 questions
1a1 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.

1b1 mark

Once the student has produced the aspirin, they need to recrystallise the product.
Explain why this is an important step.

1c1 mark

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

1d3 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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2a3 marks

A student prepared 3-nitromethyl benzoate 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

Suggest one improvement each to Step 12 and 3.

2b1 mark

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

2c2 marks

Once the mixture is recrystallised and dried the melting point of the product can be tested. The melting point of 3-nitromethylbenzoate is 78 ℃.

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

2d3 marks

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

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3a2 marks

A student prepared an ester from ethanol and ethanoic acid in the presence of concentrated sulfuric acid.

Write an equation, including state symbols, to show the preparation of the ester.

3b1 mark

The mixture that the student headed in the pear shaped flask contained 10 cm3 of ethanol, 12 cm3 of glacial ethanoic acid and 15 drops of concentrated sulfuric acid. This mixture was heated under reflux for 15 minutes.

State what is missing from the pear shaped flask.

3c4 marks

Once the ester has been formed, the reaction mixture is poured into a beaker containing a saturated solution of sodium carbonate, Na2CO3 (s).

Explain why this is a necessary step and use an equation including state symbols to demonstrate your answer.

3d5 marks

Label the following apparatus in Figure 1 that is used to separate the reaction mixture from the beaker.

8-3-aqa-a-level-chemistry-sq-q3d-easy

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4a3 marks

A student carried out a practical test to determine the identity of three unlabelled bottles containing separate solutions of an alcohol, an aldehyde and a ketone. The results are  shown in Table 1.

Table 1

 

Solution A

Solution B

Solution C

Heated under reflux with acidified K2Cr2O7

Orange to green

Orange to green

No visible change

Warmed with Tollens’ reagent

No visible change

Silver mirror observed

No visible change

 

Use Table 1 to identify each of the alcohol, aldehyde and ketone.

4b1 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. 

4c5 marks

A student is asked to identify a derivative of methyl benzoate.

Outline the steps the student should take to obtain and identify the derivative of methyl benzoate.

4d4 marks

A student reacted 10.00 g of benzoic acid with methanol and obtained 5.24 g of methyl benzoate after recrystallisation. Calculate the percentage yield of the methyl benzoate.

You should assume a 1 :1 reaction ratio. 

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5a1 mark

A student sets up a thin-layer chromatography experiment as shown in Figure 1.

Figure 1

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What error has the student made in their experimental setup?

5b1 mark

An unknown substance is analysed by thin-layer chromatography. The resulting chromatogram is shown in Figure 2.

Figure 2

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Discuss the purity of the unknown substance.

5c2 marks

A dipeptide is hydrolysed into its component amino acids. The resulting mixture is then analysed using thin-layer chromatography. The results are shown in Figure 3.

Figure 3

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Use Table 1 to name the individual amino acids that formed the original dipeptide.

Table 1

A = Lysine

B = Serine

C = Histidine

D = Tyrosine

E = Isoleucine

F = Methionine

 

5d2 marks

A chromatogram of an unknown compound is shown in Figure 4.

Figure 4

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Calculate the Rf value of the unknown compound.

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1a3 marks

Aspirin, Figure 1, can be synthesised by the acylation of 2-hydroxybenzoic acid using either ethanoyl chloride or ethanoic anhydride.

Figure 1

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Outline the mechanism for the formation of aspirin using ethanoyl chloride.

1b3 marks

In industry, it is more common to manufacture aspirin using ethanoic anhydride instead of ethanoyl chloride.

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

ii)
State two reasons why ethanoic anhydride is used in preference to ethanoyl chloride for the acylation. 
1c5 marks

A student uses the following method to make aspirin (Mr 180.158 g/mol) 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/cm3.

In your answer, you should identify the limiting reagent.

1d5 marks

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

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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, M = 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 in excess. 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 oC.

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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3a3 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.

3b3 marks

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

3c3 marks

A student has correctly identified the cyclohexanol sample.

They set up the following equipment, Figure 1, to determine the boiling point of the cyclohexanol.

Figure 1

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Explain three ways that the student could improve their equipment.

3d2 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 161oC but following distillation it was found to be 168oC.

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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4a5 marks

Propyl butanoate can be made in the laboratory by the reaction of an appropriate acid and alcohol. The ester formed is distilled off as a measure to increase the yield.

i)
Write an equation for the formation of propyl butanoate using an acid and an alcohol. 

ii)
Explain how removing the ester can help to increase the overall yield of the ester. 
4b4 marks

Ethyl ethanoate can be made in a similar fashion using ethanol and ethanoic acid. Part of the method that a student followed for this practical is given below:

  • 50 cm3 of ethanoic acid and 50 cm3 ethanol are mixed thoroughly in a 250 cm3 round bottomed flask.
  • 10cm3 of concentrated sulphuric acid is added dropwise to the mixture.
  • The mixture is well mixed and kept cool in an ice-bath.
  • When all of the acid has been added, a reflux condenser is fitted to the flask.
  • The mixture is then gently boiled over an electric heating mantle for around 30 minutes.
  • The mixture is then cooled and rearranged for distillation.

Parts of the method are in bold text. For each part, decide if it is essential and justify your answer.

4c2 marks

After distillation of approximately 50 cm3 of their crude ethyl ethanoate, the distillate is shaken in a separating funnel with 25 cm3 of sodium carbonate solution. At intervals, the pressure inside the separating funnel is released. After shaking, the lower aqueous layer is discarded.

25 cm3 of calcium chloride solution is then added to the separating funnel and the mixture is shaken. The lower layer is again discarded after shaking.

Suggest why sodium carbonate and calcium chloride were used to wash the distillate.

4d3 marks

Another student prepared ethyl ethanoate using the method outlined in parts b and c. They reacted 10 cm3 of ethanol (density = 0.79 g cm-3) with excess ethanoic acid.

After purification, the student had obtained 6.90 g of ethyl ethanoate.

Calculate the student’s percentage yield.

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5a6 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.

5b3 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.

5c3 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.

5d3 marks

The structure of aspirin is shown in Figure 1.

Figure 1

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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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1a2 marks

A student is asked to investigate the purity of a number of compounds by thin-layer chromatography. Figure 1 shows the set up of their TLC plate.

Figure 1

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Explain why the student may not be able to confidently comment on the purity of the compounds tested using their TLC plate.

1b2 marks

A student is asked to identify the three amino acids in an unknown sample.

The chromatogram showing the student’s results is shown in Figure 2.

Figure 2

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Table 1 shows the Rf data that the student used to identify two of the unknown amino acids as lysine and leucine.

Table 1

Amino acid

Alanine

Cysteine

Leucine

Lysine

Methionine

Threonine

Valine

Rf value

0.38

0.4

0.73

0.14

0.55

0.35

0.61

 

Explain two additions or improvements to the method that would allow the student to confidently identify each of the three amino acids.

1c4 marks

A student is asked to use thin-layer chromatography to identify the components of a commercially available analgesic tablet. The possible identities of the tablet components are shown in Figure 3.

Figure 3

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Draw a labelled diagram to show how the student could identify the components of the commercially available tablet using one TLC plate.

1d3 marks

A student completed the chromatography experiment, from part (c), to identify the components of a commercially available analgesic tablet.

  1. One of each tablet was crushed into a fine powder
  2. 1 g of each powdered tablet was then dissolved in a small amount of ethanol
  3. These samples were then spotted on to the TLC plate
  4. The TLC plate was then placed into a beaker with an ethyl acetate solvent
  5. A lid was placed on the beaker
  6. The plate was allowed to run until the solvent front was 0.5 cm from the top
  7. The plate was removed, solvent front marked and left to dry
  8. The components were then located

The student’s chromatogram is shown in Figure 4.

Figure 4

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The student made two conclusions:

  1. The commercially available tablet contained caffeine and paracetamol.
  2. The commercially available tablet would not be as effective a painkiller as aspirin or ibuprofen.

Discuss the student’s second conclusion.

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2a4 marks

Before a student can prepare aspirin, the salicylic acid used must be pure. Describe how the student can purify the salicylic acid and explain why each step is required.

2b4 marks

The student prepared a sample of aspirin by following the two stages outlined as follows:

Stage 1

Add a known mass of the purified salicylic acid into a dry pear shaped flask and add an excess of 5 cm3 of ethanoic anhydride followed by 8 drops of concentrated phosphoric acid. Attach a condenser to the flask and in the fume cupboard warm the mixture with swirling until the solid has dissolved and warm for another 5 minutes

Stage 2

Carefully add 5 cm3 of cold water to the solution and stand the flask in cold water for precipitation to occur. Filter off the product and wash the product with a little cold water. Transfer to a watch glass and leave to dry overnight. Once dry, weigh your product and record the mass.

Suggest four improvements to the method the student followed.

2c2 marks

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

2d3 marks

The reaction for the preparation of aspirin is shown in Figure 1.

Figure 1

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The average percentage yield for the industrial synthesis of aspirin is around 79.2%. If 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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3a2 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.

3b2 marks

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

3c2 marks

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

3d7 marks

Methyl benzoate is an ester that can be formed from benzoic acid. Outline the mechanism for the alkaline hydrolysis of this ester showing the formation of the salt and alcohol produced.

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4a4 marks

Methyl salicylate, C6H4(OH)COOCH3, shown in Figure 1 is also known as oil of Wintergreen. It is a natural product of many species of plants. Methyl salicylate may be used by plants as a pheromone to warn other plants of pathogens such as tobacco mosaic virus. Methyl salicylate is also used in deep heating liniments and in small amounts as a flavoring agent.

Figure 18-3-aqa-a-level-chemistry-sq-q4a-hard

 

Methyl salicylate was formed by the following method

  • 6 g of salicylic acid and 15 cm3 of methanol were added to a pear shaped flask
  • 2 drops of sulfuric acid were added dropwise
  • When all of the acid is complete a reflux condenser was fitted forming an airtight seal and the reaction is heated using an electric heating mantle at 60 ℃ for 30 minutes
  • Once cool 30 cm3 of distilled water and 15 cm3 of ethyl ethanoate were added
  • The mixture is transferred to a separating funnel and sodium carbonate solution is added
  • Calcium chloride solution was added and the mixture was shaken
  • The pressure was released 15-20 times
  • The lower layer was discarded
  • The ester was run into a clean, dry flask containing anhydrous calcium chloride and swirled
  • The ester was filtered into a clean, dry flask

Explain the importance of the parts of the method underlined.

4b1 mark

Suggest how an airtight seal could be formed between the reflux condenser and the round bottom flask

4c3 marks

A student performed the synthesis of wintergreen oil and recorded the data in Table 1.

Table 1

Mass salicylic acid, C7H6O3

0.1150 g

Volume methanol

5.0 cm3

Volume 9.0 M H2SO4

3 drops

Mass test tube

28.9671 g

Mass test tube and product

29.0561 g

 

Methanol has a density of 0.792 g cm-3

Find the limiting reagent for the reaction and therefore the percentage yield for this reaction.

4d4 marks

Aspirin is also known as acetylsalicylic acid and can also be produced from salicylic acid.

i)
Write an equation for the ionisation of aspirin using HAsp to represent the formula of 

ii)
Calculate the Ka of aspirin if the pH is found to be 2.15 for a 0.10 mol dm-3
Include the units for Ka

iii)
Suggest why the answer to part (ii) is not close to the true value. 

 

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5a6 marks

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.

5b4 marks

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

5c2 marks

A student outlines the mechanism for the electrophilic substitution for the formation of 4-nitrotoluene

Figure 1
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Identify the errors that the student has made

5d1 mark

According to a data book, the melting point of methyl 3-nitrobenzoate is 78 ℃.

A student tested the product produced in part (a) and found the melting point to be lower than expected.         

Explain how the student can obtain a more accurate value.

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