Mass Spectrometry (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.

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

Compound X was analysed using both techniques. Fig. 3.1 shows the infrared spectrum of compound X and Fig. 3.2 shows the mass spectrum of compound X.

Fig. 3.1 

Fig. 3.2 

Table 3.1

Use Fig. 3.1, Fig. 3.2 and Table 3.1 to suggest what evidence there is to support that compound X is a hydrocarbon.

Explain how the mass spectrum supports that compound X has a molecular formula of C₄H₁₀.

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

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 1.1 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.

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. 3.1 shows the mass spectrum of compound F.

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

Fig. 3.2 shows the infra-red spectrum of compound F. 

Table 3.1 shows the characteristic infrared absorption frequencies for some selected bonds.

Complete Table 3.2 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.