Analytical Techniques (OCR A Level Chemistry)

Exam Questions

3 hours37 questions
1a4 marks

This question is about infrared spectroscopy.

State the four different modes of vibration in molecules that have characteristic frequencies of vibration.

1b1 mark

State why molecules of oxygen, chlorine and hydrogen are each inactive in infrared spectroscopy.

1c3 marks

Complete the table by adding the expected wavenumber range for the given bonds. 

Bond Wavenumber / cm-1
C–O in a carboxylic acid  
O–H in an alcohol  
Cidentical toN in a nitrile  

1d1 mark

2-methylpropanal has the structural formula (CH3)2CHCHO. 

2-methylpropanal
 

Identify a peak that suggest that this infrared spectrum belongs to 2-methylpropanal.

 
4-7_q1d-ocr-a-as--a-level-easy-sq
1e1 mark

2-methylpropanal has different C–H peaks in its infrared spectrum due to the complex interactions between vibrating and bending bonds. 

rXI9kHBA_4-7_q1e-ocr-a-as--a-level-easy-sq
  

The peak at around 1400 cm-1 is an absorption due to the C–H bond in the aldehyde group.

 
4-7_q1e-ocr-a-as--a-level-easy-sq
 

State the name given to the region, on the infrared spectrum, that this peak is found in.

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2a1 mark

This question is about the underlying chemistry of mass spectrometry.

Give the state of matter that all samples are in before they are bombarded with high-speed electrons.

2b
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2 marks

Write the equation for the formation of the molecular ion of octane and predict its m/z value.

2c2 marks

The mass spectrum of butane shows a molecular ion peak at m/z = 58.0. 

Explain why there is also a smaller peak at m/z = 59.0 on the mass spectrum of butane.

2d1 mark

State, with a reason, whether butane or octane will have a higher [M+1] peak.

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3a3 marks

This question is about the application of different techniques to determine the structure of an unknown organic compound, Q.

State three types of analytical data that can be used to deduce the structure of an unknown organic compound, Q.

3b
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3 marks

Chemical analysis of an unknown organic compound, Q, gave the percentage composition by mass C: 66.7%; H: 11.1%; O: 22.2%. 

Calculate the empirical formula of the unknown organic compound, Q.

(Ar: C = 12.0; H = 1.0; O = 16.0)
3c5 marks

The following peaks were observed in the mass spectrum of the unknown organic compound, Q

m/z Interpretation
73.0  
72.0  
54.0  
28.0  
14.0  
 

Complete the table by interpreting the peaks in the mass spectrum.

3d2 marks

Student A correctly suggests that the unknown organic compound Q is cyclobutanol.

 
cyclobutanol
 

Student B incorrectly suggests that the unknown organic compound Q contains an alcohol and an alkene functional group.

State how the infrared spectrum of the unknown organic compound Q could prove that student A is correct.

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4a
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2 marks

There are three possible isomers, AB and C, with the molecular formula C3H8O.

Isomers A and are positional isomers of each other.

Isomer is a functional group isomer of A and B.

A student uses the following equation to calculate that the percentage by mass of carbon in all three isomers is 60.0%. 

Percentage space by space mass equals fraction numerator total space mass space of space element over denominator overall space mass space of space compound end fraction cross times 100
 

Calculate the percentage by mass of hydrogen and oxygen in all three isomers.

4b2 marks

The infrared spectrum of isomer C does not have a peak in the region of 3200 - 3600 cm-1.  

4-7_q4b-ocr-a-as--a-level-easy-sq
 
i)
Use the infrared spectrum to suggest a structure for isomer C.
 
[1]
ii)
Suggest which bonds the highlighted peak at around 1130 cm-1 could represent.
 
[1]
4c1 mark

Isomers A and B both have a peak at 3200 - 3600 cm-1 in their infrared spectra. 

State two possible chemical names for isomers A and B.

4d
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2 marks

Isomer A has a major peak on its mass spectrum at m/z = 31. 

Suggest, with a reason, which compound is isomer A.

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5a2 marks

Recent advances in technology, alongside a landmark decision by the European Council, could see breathalyser locks being fitted to all new cars.

In a measure to reduce drink-driving, this would mean that the car will not start without a negative test from the driver.

Explain how a breathalyser works.

5b1 mark

The mass spectrum of ethanol shows a peak at m /z = 15 for a methyl ion, CH3+

Complete the chemical equation to show the formation of this ion.

 
C2H5OH+ → CH3+ + _____
5c
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3 marks

State and explain three other peaks that would be expected in the mass spectrum of ethanol. 

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6a
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3 marks

An analytical chemist was provided with the unbranched compound, K.

Analysis of compound K obtained the following results:

Type of analysis Evidence
Elemental analysis C = 54.5%, H = 9.1%, O = 36.4%
Mass spectrometry Molecular ion peak at m/z = 88.0
Peak at m/z = 45.0
Infrared spectroscopy Sharp absorption at 1720 cm-1 
Broad absorption at 3000 cm-1 

 
Calculate the empirical formula of compound K

(Ar: C = 12.0; H = 1.0; O = 16.0)
6b
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2 marks

Use the mass spectrometry evidence to calculate the molecular formula of compound K.

6c
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3 marks

Explain how the evidence suggests that compound K is a carboxylic acid.

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1a3 marks

The following two compounds will both react with sodium hydroxide to produce the same product. 

MAXzm6yW_2-halo-3-methylbutanes-displayed

i)

Draw the structure of the product formed when the compounds react with dilute aqueous sodium hydroxide. 

ii)

State which compound would be converted into the product more quickly, if the reaction was done at the same temperature with the same concentration of sodium hydroxide. Explain your answer.

1b1 mark

State how you could use infrared spectroscopy to determine that the reaction in part (a) had taken place.

1c2 marks

1-chloro-3-methylbutane underwent the same reaction as is stated in part (a). A structural isomer of the product in part (a) was formed. 

Name the product of the reaction following the IUPAC rules and give the structural formula of the product.

1d5 marks

A group of students are trying to determine the identity of three isomeric organic compounds, R, S and T

All three compounds were prepared from the dehydration of 2-chlorobutane, have the same molecular formula, and will all decolourise bromine water. S and T have the same structural formula. 

Use this information to draw the displayed formula of R, S and T.

Suggest the type of isomerism which is shown between R and S and between S and T.

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2a
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2 marks

A student wanted to distinguish between the following three compounds using mass spectrometry. 

xyz

Explain why determining the exact mass using mass spectrometry would not help in distinguishing between the samples of X, Y and Z.

2b2 marks

Compounds X, Y and Z were analysed using IR spectroscopy. The spectrum of one of the compounds is shown below.

3c-ir-spec

Identify which of the three compounds X, Y or Z this spectrum belongs to. Explain your reasoning.

2c2 marks

Explain why infrared spectroscopy alone could not be used to distinguish between compounds X and Y.

2d
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3 marks

Explain how mass spectrometry can distinguish between compounds X and Y.

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3a2 marks

Infrared spectroscopy is a common analytical tool used in chemistry, to identify or distinguish between different organic compounds. 

State how an IR spectrum could be used to prove that a ketone was butan-2-one. 

3b2 marks

Breathalysers are used to test whether someone has been drinking alcohol while driving. Early models of breathalysers used chemicals which changed colour during an oxidation reaction. 

Inside the breathalyser, ethanol was oxidised to ethanoic acid using the same reagents and the same oxidation reaction which would be completed in a lab.  

State the reagents which are used to cause this oxidation and state the colour change which would be observed.

3c3 marks

The mass spectrum of the ketone, hexan-2-one has a large fragment ion at m/z = 43.

i)

Suggest two possible structures for this fragment ion.

ii)

Write an equation for the formation of each structure from the molecular ion.

3d
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7 marks

An organic molecule, J, has the following composition by mass. 

  • 62.1% carbon
  • 10.3% hydrogen 
  • 27.6% oxygen

In the mass spectrum of J, the molecular ion peak has a value of m/z = 116.

i)

Use this information to determine the molecular formula of J

ii)

State how an IR spectrum could be used to prove that compound J is a carboxylic acid.

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4a2 marks

Infrared spectroscopy and masss spectrometry are used in the search for extra-terrestrial organic compounds.

Compound X was analysed using both techniques. The infrared spectrum and mass spectrum are shown below.

compound-x-ir-spec

compound-x-mass-spec 

What evidence is there to support that compound X is a hydrocarbon?

4b1 mark

How does the mass spectrum support that compound X has a molecular formula of C4H10?

4c1 mark

Draw the displayed formulae of two possible isomers for compound X.

4d4 marks

Use the fragment ions at peaks at m/z 15, m/z 29 and m/z 43 in the mass spectrum to name compound X. Explain your reasoning.

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5a
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3 marks

An unknown saturated organic compound contains carbon, hydrogen and oxygen only.

Analysis of 1.00 g of the unknown compound showed it to contain 0.133 g of hydrogen and  0.600 g of carbon.

The infrared spectrum and mass spectrum of the unknown compound are shown below.

compound-x-2-ir-spec

compound-x-2-mass-spec

Calculate the empirical and molecular formulae for the unknown compound.

5b2 marks

Using the infrared spectrum, suggest two structural formulae for the unknown compound. Explain your reasoning.

5c2 marks

Using the mass spectrum, suggest a structure for the unknown compound. Explain your reasoning.

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1a
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3 marks

An unknown compound, A, is analysed and shown to have a composition of 69.7% carbon, 18.6% oxygen and the remainder is hydrogen.

The following incomplete mass spectrometry and IR spectroscopy data are collected.

Mass spectrum

  • M+1 peak at m/z = 87
  • Fragment ion peak at m/z = 28

IR spectrum 

  • Sharp peak at 1720 cm-1 
  • No broad peak around 3200 cm-1 

Determine the molecular formula of compound, A.

1b3 marks

Use the incomplete mass spectrometry and IR spectroscopy data to identify two possible isomers for compound A

Your answer should include an explanation proving your two possible isomers and disproving one other isomer. 

1c3 marks

A mass spectrum peak at m/z = 29 could be used to distinguish between the two remaining isomers. 

Identify the fragment responsible for this peak and explain how it can be used to distinguish between the two isomers.

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2
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6 marks

This question is about the determination of the structure of an unknown compound, B.

Compound B is a trans stereoisomer that is a useful intermediate in organic synthesis. 

Analysis of compound B shows it to contain 68.6 % carbon, 8.6 % hydrogen and 22.8 % oxygen.

The infrared spectrum of the compound B is shown in Figure 1.

Figure 1

7

In the mass spectrum, shown in Figure 2, the peak with the greatest relative intensity is caused by the loss of a functional group from the molecular ion of the compound B.

Figure 2

8

Determine the structure of compound B. Explain your reasoning and show your working.

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3a3 marks

An unknown organic compound, C, is analysed by mass spectrometry and IR spectroscopy and the following data is collected.

Mass spectrum:

4-7_q3a-ocr-a-as--a-level-hard-sq

IR spectrum:

4-7_q3a2-ocr-a-as--a-level-hard-sq

Using the molecular ion peak at m/z = 55 and the IR spectrum, suggest one possible identity of the unknown organic compound C. Explain your answer.

3b1 mark

The mass spectrum of compound C shows an M+1 peak at m/z = 56. 

Suggest two ways that this peak could be formed.

3c
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2 marks

Calculate, to three significant figures, the expected elemental composition of an 8.0 g sample of compound C

3d2 marks

Compound C undergoes a reaction to form compound D. The IR spectrum of compound D is shown: 

4-7_q3d-ocr-a-as--a-level-hard-sq 

State the reagents required for this reaction. Use the IR spectrum of compound D to justify your answer.

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49 marks

Isomers D, E and F, are analysed, using a combination of techniques. Some of the analytical data and other known information is shown below.

Elemental analysis by mass

  • Carbon, hydrogen and oxygen in the ratio of 6 : 1 : 2

Mass spectrum

  • Molecular ion peak at m/z = 72.0

IR spectrum

  • The IR spectra of all three isomers shows a peak at 3500 cm-1 but only D and E contain a peak at 1650 cm-1 

Other known information

  • D is the only branched carbon chain
  • E is the E-isomer of a pair of stereoisomers

Determine the displayed formulae of isomers D, E and F, showing all your reasoning. 

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5a3 marks

2-Bromopropane was heated under reflux with concentrated potassium hydroxide dissolved in a mixture of water and ethanol.

The organic products, G and H, formed in this reaction belong to different homologous series.

The mass spectrum of G shows peaks at m/z = 45 and at m/z = 60.

Identify compound G, showing your reasoning.

5b2 marks

The infrared spectra of products G and H are shown below. 

Product G
 
propan-2-ol-ir-spectrum
 
Product H
 
product-h-ir-spectrum
 

Explain how each spectrum shows the homologous series to which each product belongs.

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6a2 marks

This question is about analytical techniques.

An alcohol can be prepared by hydrolysing the haloalkane C2H5CH(Br)CH3 with aqueous sodium hydroxide. The C-Br bond is labelled on the IR spectrum below.

2-bromobutane-ir-spectrum

Outline how IR spectroscopy will change as a result of the above reaction.

6b1 mark

The mass spectrum of 2-methylpropan-1-ol is shown below. 

2-methylpropan-1-ol-mass-spectrum
 

Determine which ion is responsible for the peak with the greatest relative intensity.

6c
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4 marks

The IR spectrum of butanal is shown below. 

butanal-ir-spec
 

Explain why this spectrum cannot be butanoic acid, but-1-ene or butan-1-ol. 

6d2 marks

Clenbuterol, shown below, is a performance-enhancing drug that is believed to increase short-term work rate and cardiovascular output. 

clenbuterol

Determine the m/z value of the molecular ion, M+, of clenbuterol, giving your reasoning.

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7a
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3 marks

An alcohol I is analysed, using a combination of techniques. The analytical data is shown below.

Elemental analysis by mass

  • C = 68.2%, H = 13.6%, O = 18.2%

Mass spectrum

  • Molecular ion peak at m/z = 88.0

Determine the molecular formula of alcohol I.

7b1 mark

Alcohol I oxidises to form a carboxylic acid. State what information this gives about alcohol I.

7c4 marks

Draw the displayed formulae and give the systematic names of four possible structural isomers that could be alcohol I.

7d1 mark

The mass spectrum of alcohol I has a major peak at m/z = 45. 

Draw the structure of the species that could give this peak.

7e2 marks

Alcohol I has a branched chain. Identify alcohol I, showing your reasoning.

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