CAIE A-Level Chemistry A2 37 Analytical Techniques Questions

Practise interpreting chromatograms and NMR spectra to identify components, compare separations and justify structures from peaks, Rf values and retention data.

Covered syllabus points

37.1 Thin-layer chromatography 37.2 Gas / liquid chromatography 37.3 Carbon-13 NMR spectroscopy 37.4 Proton NMR spectroscopy

[Maximum number: 3]

A student uses thin-layer chromatography (TLC) to analyse a mixture containing different metal cations. The student repeats the experiment using different solvents.

Fig. 6.1 shows the chromatogram obtained by the student using water as a solvent.

Fig. 6.1

(a)

Suggest a compound that could be used as the stationary phase in this experiment.

[ 1 ]

$\mathrm{SiO}_{2}$ OR $\mathrm{Al}_{2} \mathrm{O}_{3}$ OR silica OR alumina OWTTE

(b)

Table 6.1 shows the $R_{\mathrm{f}}$ values for different metal cations when separated by TLC using water as a solvent.

Table 6.1

Suggest the identity of the cation that causes the spot at M in Fig. 6.1.

Explain your answer.

[ 1 ]

$\mathrm{Cd}^{2+}$ AND $R_{\mathrm{f}}$ of M(=2 / 5)=0 . 4 0 / same $R_{\mathrm{f}}$ as in the Table 6.1

(c)

The student repeats the experiment using butan-1-ol as a solvent.

The metal cations do not travel as far up the TLC plate in this experiment.
Suggest why the metal cations do not move as far up the TLC plate with butan-1-ol as a solvent.

[ 1 ]

[Maximum number: 8]

Lactic acid, $\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}$ , is the only monomer needed to form the polymer polylactic acid, PLA.

(a)

The proton $\left({ }^{1} \mathrm{H}\right) \mathrm{NMR}$ spectrum of $\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}$ in $\mathrm{CDCl}_{3}$ is shown in Fig. 8.1. The proton NMR chemical shift ranges are shown in Table 8.1.

Lactic acid

Table 8.1

[ 5 ]

(i)

Use Fig. 8.1 and Table 8.1 to complete Table 8.2.

Table 8.2

[ 3 ]

(ii)

Name the substance responsible for the peak at $\delta=0.0$ .

[ 1 ]

(iii)

Explain why $\mathrm{CDCl}_{3}$ is a better solvent than $\mathrm{CHCl}_{3}$ for use in proton NMR .

[ 1 ]

$\mathrm{CDCl}_{3}$ will not give an absorption / peak OR

$\mathrm{CHCl}_{3}$ will give an absorption / peak [1]

(b)

An impure sample of $\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}$ contains pentan-3-one as the only contaminant. The mixture is analysed using gas/liquid chromatography. The pentan-3-one is found to have a longer retention time than the lactic acid.

[ 3 ]

(i)

Explain what is meant by retention time.

[ 1 ]

(ii)

Suggest suitable substances, or types of substances, that could be used as the mobile and stationary phases.
mobile phase
stationary phase

[ 1 ]

(iii)

Describe how the percentage composition of the mixture can be determined from the gas/liquid chromatogram.

[ 1 ]

area of peak divided by total area of all peaks $\times 100 \%$ [1]

[Maximum number: 3]

(a)

Deduce the number of peaks that would be present in the carbon-13 NMR spectrum of benzophenone.
number of peaks

[ 1 ]

(b)

Identify two different environments of carbon atom that would result in different chemical shift ranges in this carbon-13 NMR spectrum of benzophenone.

Table for Question (b) — CAIE A-Level Chemistry A2

[ 2 ]

environment of carbon atom	chemical shift range (δ)
carbonyl / RCOR	190–220
arene / benzene	110–160

[Maximum number: 5]

Amino acids are molecules that contain $-\mathrm{NH}_{2}$ and -COOH functional groups.
Glycine, $\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{COOH}$ , is the simplest stable amino acid.

(a)

Compound V is another amino acid.

The proton $\left({ }^{1} \mathrm{H}\right)$ NMR spectrum of V shows hydrogen atoms in five different environments, a , b , c, d and e, as shown in Fig. 6.2.

Fig. 6.2

Table 6.1

[ 5 ]

(i)

Complete Table 6.2 for the proton $\left({ }^{1} \mathrm{H}\right)$ NMR spectrum of V taken in $\mathrm{CDCl}_{3}$ .

Table 6.2

[ 4 ]

proton | a | b | c | d | e

$\delta$ | 9.0-13.0 | 6.0-9.0 | 2.3-3.0 | 3.2-4.0 | 1.0-5.0

splitting | singlet | multiplet | triplet | triplet | singlet

any three [1] any five [2] any seven [3] all nine [4]

(ii)

Complete Table 6.3 by placing a tick ( $\checkmark$ ) to indicate any protons whose peaks are still present in the proton $\left({ }^{1} \mathrm{H}\right)$ NMR spectrum of V taken in $\mathrm{D}_{2} \mathrm{O}$ .

Table 6.3

[ 1 ]

proton & a & b & c & d & e

present in $\mathrm{D}_{2} \mathrm{O}$ & & & & &

All question bank results loaded

Covered syllabus points

Question 1

Question (a)

Question (b)

Question (c)

Question 2

Question (a)

Question (i)

Question (ii)

Question (iii)

Question (b)

Question (i)

Question (ii)

Question (iii)

Question 3

Question (a)

Question (b)

Question 4

Question (a)

Question (i)

Question (ii)