Analytical Techniques (CIE A Level Chemistry)

Compound X contains atoms of carbon, hydrogen and oxygen only.

The mass spectrum of X is recorded. Information about the two peaks with m / e greater than 100 is shown in Fig. 1.1.

Fig. 1.1

A molecule of X contains 6 carbon atoms.

Demonstrate that this is correct using information from Fig 1.1. Show your working.

Suggest the molecular formula of X using information from Fig. 1.1.

Suggest the molecular formula of the fragment of X at m / e = 31.

Fig 1.2 shows the infra-red spectrum of X.

The data in Fig 1.2, Table 1.1 and your answer to (b) should be used in answering this question.

Identify the functional group present in X. Explain your answer.

The mass spectrum of magnesium is shown in Fig. 2.1. 

From the mass spectrum, complete Table 2.1 with the relative abundances of the three isotopes.

Use your values in (a) to calculate the relative atomic mass, A_r, of magnesium to two decimal places.

A_r (Mg) = ...........................

Give the full electronic configuration of the magnesium ion, ²⁶Mg²⁺.

Boron has two naturally occurring isotopes, ¹⁰B and ¹¹B. The relative atomic mass of boron is 10.8

Use this information to deduce the relative abundance of each isotope.

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

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

Compounds X, Y and Z were analysed using IR spectroscopy. The spectrum of one of the compounds is shown in Fig 3.1.

Fig 3.1

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

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

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

The isomers A and B, C₅H₁₀O, both form an orange precipitate when reacted with 2,4-DNPH.

A is unbranched and reacts with alkaline aqueous iodine to produce a yellow precipitate.

B does not react with alkaline aqueous iodine. It contains a chiral centre and produces a silver mirror when warmed with Tollens’ reagent.

Draw the structure of the yellow precipitate produced by the reaction between A and alkaline aqueous iodine.

C and D are isomers with the molecular formula C₃H₆O₂ . The infra-red spectra of isomers C and D are shown in Fig. 4.1 and Fig. 4.2.

Fig. 4.1

Fig. 4.2

Table 4.1

Using Table 4.1, identify the bonds responsible for the principal peaks above 1500 cm^–1 in each spectrum.

spectrum of C ............................................................................................

spectrum of D ............................................................................................

Draw possible structures of isomers C and D

Identify the ions responsible for peaks at 73, 45 and 29 in the mass spectrum for compound C.

This question is about the analysis of alcohols.

Alcohol A can be prepared by hydrolysing the halogenoalkane C₂H₅CHBrCH₃ with aqueous sodium hydroxide. The melting and boiling point information of alcohol A and C₂H₅CHBrCH₃ are given in Table 1.1.

Write an equation for the hydrolysis of C₂H₅CHBrCH₃ with aqueous sodium hydroxide. Include state symbols in your answer

Using the information in Table 1.1, explain how alcohol A can be separated from the products identified in part (a). 

Describe two chemical tests and their expected observations to confirm the identity of the inorganic product.

The infra-red spectrum of C₂H₅CHBrCH₃ is shown in Fig. 1.1 with the C–Br bond absorption labelled.

Using Fig. 1.1 and Table 1.2, predict three main differences that would be seen between the spectra of C₂H₅CHBrCH₃ and alcohol A.

The mass spectrum of (CH₃)₂CHCH₂OH is shown in Fig. 1.2.

Identify the ion that is responsible for the peak with the greatest relative intensity.

An unknown compound, B, 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 / e = 87.0
• Fragment ion peak at m / e = 28

Iinfra-red spectrum 

• Sharp peak at 1705 cm^-1 
• No broad peak around 3200 cm^-1 

Calculate the molecular formula of compound B.

Using Table 2.1 and the incomplete mass spectrometry and infra-red spectroscopy data, identify two possible isomers for compound B.

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

A mass spectrum peak at m / e = 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.

Unknown organic compound C is analysed by mass spectrometry and infra-red spectroscopy. The mass spectrum is shown in Fig. 3.1 and the infra-red spectrum is shown in Fig. 3.2.

Fig 3.1

Fig 3.2

Deduce one possible identity for compound C. Explain your answer.

In your answer, use the molecular ion peak at m / e = 55, relevant infra-red absorptions in the region above 1500 cm^-1 and Table 3.1.

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

Suggest two ways that this peak could be formed.

Calculate the expected elemental percentage composition by mass of an 8.0 g sample of compound C. Give your answers to significant figures. 

Compound C undergoes a reaction to form compound D. The IR spectrum of compound D is shown in Fig. 3.3.

State the reagents required for this reaction. In your answer, use Table 3.1 and identify any relevant absorptions in the infra-red spectrum to justify your answer. 

Alcohol E 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 / e = 88.0
• Major ion peak at m / e = 45.0

Determine the molecular formula of alcohol E. 

Alcohol E oxidises to form a carboxylic acid. Suggest what information this gives about the structure of alcohol E.

The mass spectrum of alcohol E has a major peak at m / e = 45.

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

Alcohol E has a branched chain.

Identify the isomer from part (d) that is alcohol E. Explain your answer. 

The term carbohydrate is often associated with sugars or suggests a compound that contains carbon, hydrogen and oxygen. It was previously used in chemistry to define any compound of the general formula C_m(H₂O)_n.

Compound F contains 6.98 % hydrogen and 37.21 % oxygen by mass. The remainder of the mass is carbon.

Fig. 5.1 shows the mass spectrum of compound F.

Using your answer to part (a)(i), deduce the molecular formula of compound F.

Fig. 5.2 shows the infra-red spectrum of compound F. 

Table 5.1 shows the characteristic infrared absorption frequencies for some selected bonds.

Complete Table 5.2 with the bond responsible for each of the principal absorptions seen in the infra-red spectrum of compound F.

Compound F is a straight-chain molecule.

Using all of the available information including your answers from parts (b) and (c), draw the displayed formulae and give the systematic names of the three possible isomers that could be compound F.

Compound F is the stereoisomer with the lowest boiling point.

Using the isomers identified in part (d), give the name of compound F. Explain your answer.