Analytical Techniques (OCR A Level Chemistry)

Exam Questions

4 hours36 questions
1a4 marks

This question is about thin layer chromatography.

Name the two phases of chromatography and give one example of a chemical used for each phase.

1b3 marks

State three methods or chemicals that can be used to locate or make the spot of a non-coloured compound visible.

1c3 marks

Thin layer chromatography is used to check if an unknown food colouring contains a banned colouring.  

Draw a labelled diagram to show the experimental setup for this TLC analysis.

1d2 marks

State two factors that the rate of separation depends upon.

1e1 mark

State the equation used to calculate the unique retention factor of a compound.

1f
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3 marks

Calculate the Rf value of the compound shown in the chromatogram below.

 
tlc-calculation
 

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2a3 marks

This question is about gas chromatography.

Name the three types of chemical that gas chromatography is used for.

2b1 mark

Silica and alumina are commonly used as the stationary phase in thin layer chromatography. 

State the type of chemical that is commonly used as the stationary phase in gas chromatography.

2c2 marks

State the type of chemical that is used for the mobile phase in gas chromatography. You should include at least one specific example in your answer.

2d4 marks

Results in gas chromatography are based on retention time. 

i)
Define retention time.
[1]
 
ii)

 State three factors that retention time depends upon.

[3]

2e1 mark

State what the relative size or area under the peak on a gas chromatogram tells you.

2f3 marks

Gas chromatography is carried out using a polar stationary phase and argon as the carrier gas.
Use the chromatogram to answer the following questions.

 
5
 
i)
Which compound has the lowest retention time?
 
[1]
ii)
Which compound is the least polar?
 
[1]
iii)
Which compound has the greatest interaction with the stationary phase?
 
[1]

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3a1 mark

Simple test-tube reactions can be used to distinguish between organic compounds.

For example, the following can be used to distinguish between but-1-ene and cyclobutane:

  • Reagent = bromine water
  • Observation with but-1-ene = bromine water changed from orange to colourless
  • Observation with cyclobutane = no visible change to the bromine water

Each of the following pairs of organic compounds are missing the reagent or an observation. Complete the information for each pair of organic compounds. 

Propan-2-ol and 2-chloropropane.

 
  • Reagent = ............................................................
  • Observation with propan-2-ol = colour change from orange to green
  • Observation with 2-chloropropane = no visible change

3b1 mark

Phenol and cyclohexene.

 
  • Reagent = bromine water
  • Observation with phenol = bromine water decolourises and a white precipitate forms
  • Observation with cyclohexene = ............................................................

3c1 mark

Pentanoic acid and pentan-2-one. 

  • Reagent = ............................................................
  • Observation with pentanoic acid = bubbles of gas formed
  • Observation with pentan-2-one = no visible reaction

3d1 mark

Suggest, with a reason, why Brady's reagent (2,4-dinitrophenylhydrazine) cannot be used to distinguish between propanal and propanone.

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4a2 marks

This question is about the NMR analysis of various organic compounds.

Name and draw the structure of the chemical that is commonly used as a standard in NMR spectroscopy.

4b3 marks

Pentane, compound A, has the chemical formula C5H12

2-methylbutane, compound B, and 2,2-dimethylpropane, compound C, are isomers of pentane.

 
labelled-isomers-of-pentane
 

State the number of hydrogen peaks that would be expected in the 1H-NMR spectrum of each isomer.

4c3 marks

C13 NMR is also commonly used to distinguish chemicals.
State the number of different carbon environments in compounds A, B and C.

4d3 marks

Explain why the methyl groups in 2-methylbutane, compound B, give a doublet splitting pattern while the methyl groups in 2,2-dimethylpropane, compound C, give a singlet splitting pattern.

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5a2 marks

This question is about the spectroscopy of benzene and some of its methylated derivatives.

Benzene is analysed using 13C-NMR spectroscopy.  

Suggest why benzene has only one peak in its spectrum and suggest where that peak would appear.

 
5b1 mark

The displayed formula of methylbenzene is shown in Figure 1.

 
Figure 1
 
methylbenzene
 

State the number of 1H peaks that would appear in the low resolution 1H NMR spectrum of methylbenzene.

5c3 marks

There are three isomers of dimethylbenzene shown in Figure 2:

  • 1,2-dimethylbenzene
  • 1,3-dimethylbenzene
  • 1,4-dimethylbenzene
Figure 2
 
dimethylbenzene-isomers
 

Complete the table to show the number of expected peaks in the low resolution 1H NMR and 13C NMR spectra of the three dimethylbenzene isomers.

 
Isomer Number of peaks in 1H NMR spectrum Number of peaks in 13C NMR spectrum
1,2-dimethylbenzene    
1,3-dimethylbenzene    
1,4-dimethylbenzene    

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6a
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3 marks

This question is about deducing the structure of an unknown organic compound.

A 5.000 g sample of an unknown compound was analysed and shown to contain 3.105 g of carbon, 0.515 g of hydrogen and 1.380 g of oxygen.

The unknown compound gives a molecular ion peak at m/z = 58.0

Determine the molecular formula of the unknown compound.

6b2 marks

The infrared spectrum of the unknown compound shows a characteristic peak at 1720 cm-1

Suggest two possible identities for the unknown compound. Explain your answer.

6c2 marks

Using your answer to part (b), suggest how 13C NMR spectroscopy could be used to determine the identity of the unknown compound. 

6d6 marks

Describe and explain the splitting patterns that would be seen for each of the compounds identified in part (b).

6e2 marks

Explain why it is not necessary to use high resolution 1H NMR to distinguish between the possible structures identified in part (b).

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1a2 marks

You are provided with the following selection of organic compounds, to investigate using test-tube reactions.

abcd

State a simple test-tube reaction which could be done to distinguish between Compound B and Compound C, including the observations that could be made.

1b4 marks

State the simple test-tube reactions that could be done to distinguish between Compound A and Compound D from part (a), including the observations that could be made.

1c5 marks

Compound D can be formed from an alcohol if the correct reagents and conditions are used for the reaction. 

i)

State the reagents, conditions and any observations that would be made during the above reaction.

ii)

Give the IUPAC name of the alcohol which would need to be used and write an equation using structural formulae for the reaction.

1d3 marks

A student claims to have fully oxidised a different primary alcohol than that in part (c). The student analysed their product and obtained the following IR spectrum. 

unknown-ir-spec

Use the spectrum to suggest whether full oxidation has taken place

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2a2 marks

A student was investigating the structure of three hydrocarbons, L, M and N.

L and M are isomers.

L has two stereoisomers, but neither of the other compounds do. 

Define the term stereoisomer and explain the key feature which must be present in L for it to show stereoisomerism.

2b2 marks

The homologous series that L and M belong to have the general formula CnH2n and the homologous series of N has the general formula CnH2n+2.

State a simple test-tube reaction that could be done to distinguish between L and N, including the observations you would see.

2c2 marks

Simple test-tube reactions like those in part (b) can also be used to distinguish between the following organic compounds. 

2-bromopropane and hexane. 

State the test-tube reaction which could be used and any observations that would be seen.

2d2 marks

Simple test-tube reactions like those in part (b) and part (c) can also be used to distinguish between the following pair of organic compounds. 

Pentan-3-ol and 3-methyl-pentan-3-ol 

State the test-tube reaction which could be used and any observations that would be seen.

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3a1 mark

Column chromatography and gas chromatography work by the same principles of components travelling through the column at different rates. 

State the difference between the eluents involved in column chromatography and gas-liquid chromatography.

3b2 marks

Gas- chromatography is performed on three compounds, A, B and C. The gas chromatogram trace is shown.

gas-chromatogram-trace-abc

 

Identify if compound A, B or C has the greatest affinity for the solid phase. Explain your reasoning.

3c1 mark

Use the gas chromatogram shown in part (b), to identify the most abundant compound in the sample. Explain your reasoning.

3d2 marks

An oil tanker is travelling through the English Channel. The tanker has a slight leak which is not large enough to result in an oil slick but some oil is noticed on a beach.

Suggest how gas chromatography could be used to identify the tanker as the source of crude oil pollution on the beach.

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4a2 marks

A student is asked to identify a mixture using thin-layer chromatography. 

Draw the labelled experimental set up that the student should use to identify the number of compounds in the mixture.

4b4 marks

The student runs their thin-layer chromatography experiment and plans to determine the compounds from their Rf values.

Describe the steps that the student needs to perform to determine the identity of the compounds.

4c2 marks

The student’s results are shown.

example-chromatogram

For their measurements, the student locates the centre of each spot by estimating its rough position by eye.

Suggest an improved method to locate the centre of each spot.

4d
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2 marks

The student runs another TLC practical on a mixture of benzaldehyde and benzyl alcohol is placed on a TLC plate using 7:3 pentane / diethyl ether as a solvent. 

The student's chromatogram is shown.

example-chromatogram-to-calculate

Calculate the Rf values for both compounds in the chromatogram.

4e2 marks

Explain why the maximum Rf value of a compound cannot exceed 1.

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5a3 marks

During the production of an NMR spectrum, tetramethylsilane (TMS) is mixed with the sample.

i)

Suggest the structural formula of the standard reference chemical used for 13C NMR spectroscopy. 

ii)

State two reasons why this chemical is suitable to be used as the standard reference chemical.

5b1 mark

Predict the number of peaks in the 13C NMR spectrum of 1,3-dichlorobenzene.

5c2 marks

The structural formula of ethylbenzene is shown below.

VRwFPzIK_ethylbenzene-asterisk

i)

Predict the number of peaks in the 13C NMR spectrum of ethylbenzene 

ii)

One of the hydrogen atoms in the structure of ethylbenzene shown above is labelled with an asterisk (*). Suggest a range of δ values for the peak due to this carbon atom in the 13C NMR spectrum of ethylbenzene.

5d7 marks

Compound A contains the elements carbon, hydrogen, oxygen and nitrogen only. Compound A contains a benzene ring.

Part of the mass spectrum of A is shown below.

compound-a-ms

i)

State the molecular formula of the species that causes the peak at m / z = 76 in the mass spectrum of A.

ii)

Draw the structures of the three possible dinitrobenzene isomers of A containing a benzene ring.

iii)

The 13C NMR spectrum of compound A has four peaks. Identify the structure of A. Explain your reasoning by labelling the different carbon environments in all the structures drawn in part (ii).

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6a2 marks

Methyl cinnamate, C10H10O2, is a white crystalline solid used in the perfume industry. A simplified high resolution mass spectrum of methyl cinnamate is shown below.

methyl-cinnamate-high-res-proton-nmr

Name the compound responsible for the peak at a chemical shift of 0 ppm. State its purpose.

6b3 marks

Identify the proton environment that causes the peak at a chemical shift of 3.8 ppm by circling it on the diagram of shown. Explain your reasoning.

methyl-cinnamate-skeletal

6c2 marks

This question is about the use of 1H NMR spectroscopy to distinguish between isomers of C6H12O2.

Draw the two esters with formula C6H12O2 that each have only two peaks, both singlets, in their 1H NMR spectra. The relative peak areas are 3:1 for both esters.

6d5 marks

The high resolution 1H NMR spectrum of another isomer of C6H12O2 is shown.

c6h12o2-isomer-1h-nmr

The integration values for the peaks in the 1H NMR spectrum of this isomer are given in the table below.

Chemical shift, δ/ppm

3.8

3.5

2.6

2.2

1.2

Integration value

0.6

0.6

0.6

0.9

0.9

Splitting pattern

triplet

quartet

triplet

singlet

triplet

i)

Deduce the simplest ratio of the relative numbers of protons in each environment in the isomer.

ii)

Use the Data sheet to deduce the part of the isomer that causes the signal at δ= 3.5 and the part of the structure at the isomer that causes the signal at δ=1.2. Explain why the splitting patterns of these peaks are produced.

6e1 mark

13C NMR spectrometry can be used to analyse organic compounds. Four isomers of C6H12O2, A, B, C and D, were analysed by 13C NMR spectrometry.

c6h12o2-abcd

The 13C NMR spectrum of one of the four isomers of C6H12O2 is shown.

c6h12o2-abcd-13c-nmr

Use your data sheet to determine which of the four isomers, A, B, C or D of C6H12O2 produced the 13C NMR spectra shown.

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7a
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3 marks

Chemical analysis of an unknown compound showed it to contain 62.1% carbon, 10.3% hydrogen and the rest oxygen.

The unknown compound gives a molecular ion peak at m/z = 116.

Determine the molecular formula of the unknown compound.

7b6 marks

The infrared and 13C NMR spectra of the unknown compound are shown.

6-9-infrared

qTgAcMzM_6-9-carbon-nmr

Analyse the spectra and suggest two possible skeletal structures for the unknown compound. 

7c5 marks

One of the two possible isomers of the unknown compound produces only three peaks in its 1H NMR spectrum.

i)

Name the isomer responsible for this spectrum.

ii)

State the splitting patterns and chemical shifts of the protons for this isomer.

iii)

State the relative peak heights for protons of this isomer.

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1a
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3 marks

A chemist analyses a naturally occurring compound, P, which is found in strawberries, apples and Parmesan cheese. The percentage composition and mass spectrum of the compound are shown below.

Percentage composition by mass: C, 58.82%; H, 9.80%; O, 31.38%.

6-9_q1a-ocr-a-as--a-level-hard-sq

Determine the molecular formula of the compound. Show your working.

1b4 marks

Qualitative tests are carried out on compound P. The results are shown below.

Test Addition of H2O Na2CO3 (aq)  2,4-DNP Tollens' reagent
Observation Forms separate layers No observable change No observable change No observable change

Use these observations to analyse the potential functional groups in compound P. Explain your reasoning.

1c3 marks

The 13C NMR spectrum of compound P is shown below.

 
6-9_q1c-ocr-a-as--a-level-hard-sq

i)
Identify the structural features responsible for the peaks at 162 ppm and 63 ppm.
 

[2]

ii)
Use the information in parts (a) and (b) to determine the functional group(s) present in compound P.
 
[1]
1d3 marks

The high-resolution proton NMR spectrum of compound P is shown below.

6-9_q1d-ocr-a-as--a-level-hard-sq

Use the spectrum and the information from parts (a), (b) and (c) to deduce the structure of compound P

Explain your reasoning.

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2a3 marks

The following table shows information about the C-13 NMR spectrum of three isomers with molecular formula C8H10.

  L M N
Number of peaks 3 5 4

Deduce the structures of L, M and N

2b3 marks

Complete the table of information about the proton NMR spectra for isomers L, M and N.

  L M N
Number of peaks      
Relative Peak area      

2c3 marks

Deduce whether peak splitting would be seen in the high-resolution proton NMR spectra of each of L, M and N.

Explain your reasoning in each case.

2d2 marks

Suggest, with a reason, which of the three isomers, L, M or N would have the highest melting point.

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3a2 marks
Methamphetamine, or crystal meth as it is commonly known, is synthesised from phenyl acetone and N-methylformamide. The structure of the reactants and product are shown below.

6-9_q3a-ocr-a-as--a-level-hard-sq

Suggest the systematic names for the reactants.

3b2 marks

A sample of methamphetamine was added to a test tube containing some 2,4-DNP.

The result was the formation of a slight orange precipitate. Explain these observations

3c3 marks

The purity of a sample of methamphetamine could be determined by IR spectroscopy.

Describe two pieces of evidence to support this statement.

Use the Data Sheet to support your answer.

3d2 marks

The proton NMR spectrum of one of the substances in part a) is shown below.

6-9_q3d-ocr-a-as--a-level-hard-sq

Determine, with a reason, which substance the spectrum belongs to.

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4a3 marks

Gas chromatography is often connected to mass spectroscopy in order to analyse each compound as it exits the gas chromatography column. This combined technique is called GC-MS.

The gas chromatogram of an organic mixture is shown below. The stationary phase is a polar, high-boiling point liquid on a solid support.

ocr-a2-sq-h-gc-6-9-q4a

i)
Name an appropriate mobile phase that could be used to form this gas chromatogram.

[1]

ii)
Identify the number of compounds present in this mixture and how to identify the most polar molecule present.

[2]

4b
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6 marks

Two of the substances have very similar retention times. 

The mass spectra of the compounds revealed that they have the same molecular formulae as each other, with subtle differences in the spectra indicating structural isomerism is present. When mixed with bromine water the solutions remained orange.

ocr-a2-sq-h-mass-spec-hexane-q4
i)
Identify the molar mass and hence the molecular formula of the isomers.
[2]
ii)
Identify the fragments responsible for the signals at m/z = 29, m/z = 43 and m/z 57.
[3]
iii)
Suggest the most likely structure of the isomer responsible for this spectrum
[1]
4c3 marks

There is a small, additional peak on the far right of the mass spectrum. 

i)
Explain the presence of the small peak at m/z = 87.

[1]

ii)
Suggest the type of molecules for which this would become more pronounced. Explain your answer.

[2]

4d4 marks

The other isomer present in the original mixture was 2,3-dimethylbutane. 

i)
Suggest one way in which the mass spectrum of 2,3-dimethylbutane will differ from that shown in part b).

[1]

ii)
Suggest an alternative spectroscopic technique for distinguishing between the two isomers.

[1]

iii)
Predict which of the two isomers would have the longest retention time and explain your reasoning.

[2]

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5a2 marks

Esters can be analysed by a number of techniques.

An ester with the molecular formula C6H12O2 has the following high-resolution proton NMR data:

Proton environment Chemical shift  Ratio Splitting pattern
1 2.0 - 3.0 3 singlet
2 3.0 - 4.3 2 doublet
3 0.5 - 1.9 1 multiplet of 9
4 0.5 - 1.9 6 doublet

Draw the part of the ester responsible for the signal in proton environment 4.

5b2 marks

Deduce the full structure of the ester in part a).

5c4 marks

The ester can undergo acid hydrolysis to form two products. 

Describe laboratory tests and expected results for the presence of the functional groups in each of the hydrolysis products.

5d1 mark

A structural isomer of the ester in part a), ester B, has a proton NMR with only two peaks, both being singlets. 

Draw the skeletal formula of ester B.

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