Organic analysisAQA A-Level Chemistry: Topic test

1. 1
   A student tests an unknown liquid, W, which has the molecular formula C₅H₁₀O. Bromine water stays orange. Aqueous sodium carbonate produces no effervescence. Warming W with Tollens' reagent produces a silver mirror. Warming W with acidified potassium dichromate(VI) changes the colour from orange to green.

   (a)

   Which functional group is present in W?

   [1 mark]

   • AAlkene
   • BCarboxylic acid
   • CAldehyde
   • DKetone

   (b)

   What happens in the Tollens' test to give the silver mirror?

   [1 mark]

   • AThe aldehyde is reduced and Ag⁺ ions are oxidised
   • BThe aldehyde is oxidised to a carboxylic acid and Ag⁺ ions are reduced to silver
   • CThe aldehyde is oxidised to a ketone and Ag⁺ ions are reduced to silver
   • DThe aldehyde is dehydrated and silver ions form a complex

   (c)

   A second liquid is pentan-2-one, also C₅H₁₀O. State the observations when pentan-2-one is warmed with Tollens' reagent and with acidified potassium dichromate(VI).

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist suspects that a white solid taken from a fertiliser bag is urea, CH₄N₂O. High-resolution mass spectrometry of the solid gives a molecular ion peak at m/z 60.0323. Precise atomic masses: H = 1.0078, C = 12.0000, N = 14.0031, O = 15.9949. Other possible compounds with a relative molecular mass of 60 are C₂H₄O₂, C₃H₈O and C₂H₈N₂.

   (a)

   Which molecular formula matches the molecular ion peak at m/z 60.0323?

   [1 mark]

   • ACH₄N₂O
   • BC₂H₄O₂
   • CC₃H₈O
   • DC₂H₈N₂

   (b)

   What does the peak with the highest m/z value in the mass spectrum represent?

   [1 mark]

   • AThe most abundant fragment ion
   • BThe fragment formed by loss of one hydrogen atom
   • CThe base peak
   • DThe molecular ion, formed when a molecule loses one electron

   (c)

   Calculate the precise relative molecular mass of C₂H₄O₂ and explain whether the measurement would allow the spectrometer to distinguish it from urea.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student oxidises butan-2-ol using acidified potassium dichromate(VI) and distils off the product. The infrared spectrum of the distillate has a strong, sharp absorption at 1715 cm⁻¹ and a broad absorption centred at 3350 cm⁻¹. Data: C=O 1680–1750 cm⁻¹; O–H in alcohols 3230–3550 cm⁻¹; O–H in carboxylic acids 2500–3000 cm⁻¹; C–H 2850–3100 cm⁻¹.

   (a)

   Identify the bonds responsible for the two absorptions and state what the spectrum shows about the distillate.

   [3 marks]

   (b)

   The student records the spectrum of pure butanone for comparison. State which absorption would be absent from it, explain how the fingerprint region can be used to confirm identity, and explain why infrared spectroscopy alone cannot distinguish butanone from butanal. Name a test-tube reaction that can.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Compound E contains carbon, hydrogen and oxygen only. High-resolution mass spectrometry gives a molecular ion peak at m/z 88.0522. The infrared spectrum of E has a very broad absorption from 2500 to 3000 cm⁻¹ and a strong absorption at 1710 cm⁻¹. Precise atomic masses: H = 1.0078, C = 12.0000, O = 15.9949. Data: C=O 1680–1750 cm⁻¹; O–H in alcohols 3230–3550 cm⁻¹; O–H in carboxylic acids 2500–3000 cm⁻¹.

   (a)

   State what the peak at m/z 88.0522 represents. The molecular formula of E could be C₃H₄O₃, C₄H₈O₂ or C₅H₁₂O. Use the precise masses to show which is correct, and explain why a low-resolution mass spectrum could not decide between them.

   [6 marks]

   (b)

   E has the molecular formula C₄H₈O₂. Use the infrared data to deduce the functional group in E. Describe test-tube reactions that confirm this group and rule out an aldehyde and an alcohol. Explain how infrared spectroscopy could show that E is butanoic acid rather than its isomer 2-methylpropanoic acid.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Pent-4-en-1-ol, CH₂=CHCH₂CH₂CH₂OH, is a colourless liquid. A student carries out three test-tube reactions on it: shaking it with bromine water, warming it gently with acidified potassium dichromate(VI) and distilling off the product as it forms, and then warming the distillate with Tollens' reagent.

   (a)

   What is observed when pent-4-en-1-ol is shaken with bromine water?

   [1 mark]

   • AThe mixture stays orange
   • BThe mixture turns colourless
   • CA silver mirror forms
   • DThe mixture turns green

   (b)

   Which compound is formed by gentle warming with acidified potassium dichromate(VI) and immediate distillation?

   [1 mark]

   • ACH₂=CHCH₂CH₂COOH
   • BCH₃CH₂CH₂CH₂CHO
   • CCH₂=CHCH₂CH₂CHO
   • DCH₂=CHCH₂COCH₃

   (c)

   State the observation when the distillate is warmed with Tollens' reagent and explain what it shows about the distillate and the original alcohol.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A sample of colourless gas from an unlabelled cylinder is either propane, carbon dioxide, dinitrogen monoxide or ethanal. High-resolution mass spectrometry gives a molecular ion peak at m/z 44.0624. Precise atomic masses: H = 1.0078, C = 12.0000, N = 14.0031, O = 15.9949.

   (a)

   Which gas is in the cylinder?

   [1 mark]

   • ACarbon dioxide, CO₂
   • BDinitrogen monoxide, N₂O
   • CEthanal, C₂H₄O
   • DPropane, C₃H₈

   (b)

   Which statement about a low-resolution mass spectrum of these four gases is correct?

   [1 mark]

   • AAll four would show a molecular ion peak at m/z 44
   • BOnly propane would show a peak at m/z 44
   • CEach gas would give a peak at a different whole number
   • DOnly the gases containing oxygen would give a molecular ion peak

   (c)

   Calculate the precise relative molecular mass of N₂O and explain whether a high-resolution spectrometer could distinguish it from propane.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   The Earth's surface, warmed by the Sun, emits infrared radiation. Atmospheric gases including carbon dioxide, methane and water vapour absorb some of this radiation, and this contributes to global warming.

   (a)

   Explain how the absorption of infrared radiation by these gases leads to warming of the atmosphere.

   [3 marks]

   (b)

   Name the bond in each of carbon dioxide, methane and water vapour that absorbs infrared radiation. A scientist reports that the concentration of methane in the atmosphere is rising. Explain why this increases global warming.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A forensic chemist analyses a liquid, L, that contains carbon, hydrogen and possibly oxygen. High-resolution mass spectrometry gives a molecular ion peak at m/z 72.0573. The infrared spectrum has a strong absorption at 1715 cm⁻¹ and no absorption between 2500 and 3550 cm⁻¹. L is one of butanal, butanone and but-3-en-1-ol. When tested, bromine water stays orange, Tollens' reagent gives no silver mirror, Fehling's solution stays blue, and acidified potassium dichromate(VI) stays orange. Precise atomic masses: H = 1.0078, C = 12.0000, O = 15.9949. Data: C=O 1680–1750 cm⁻¹; O–H in alcohols 3230–3550 cm⁻¹; O–H in carboxylic acids 2500–3000 cm⁻¹.

   (a)

   The molecular formula of L could be C₃H₄O₂, C₄H₈O or C₅H₁₂. Use the precise masses to decide which is correct, and use the infrared spectrum to state what it shows about the functional groups in L.

   [6 marks]

   (b)

   Explain how the test-tube results show that L is butanone and not butanal or but-3-en-1-ol.

   [6 marks]

   Total for question 8: 12 marks

End of questions