CourseCourseConceptConceptDocumentDocument

CAIE A-Level Chemistry A2 37.2 Gas Liquid Chromatography Questions

Practise defining phases and retention time, interpreting peaks and calculating percentage composition.

Syllabus
2028–2030
Course
Chemistry 9701
Level
A2

Exam points

  • identify an unreactive carrier gas as mobile phase and a high-boiling non-polar liquid as stationary phase
  • define retention time from injection to detection and link longer times to stronger stationary-phase attraction
  • calculate each component’s percentage as its peak area divided by total peak area, multiplied by 100

Covered syllabus points

37.2.1—And understand the terms37.2.2—Gas / liquid chromatograms37.2.3—Retention times

Question 1

[Maximum number: 3]

Lactic acid, CH3CH(OH)COOH\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}, is the only monomer needed to form the polymer polylactic acid, PLA.

Question (a)

(a)

An impure sample of CH3CH(OH)COOH\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 ]

Question (i)

(i)

Explain what is meant by retention time.

[ 1 ]

Question (ii)

(ii)

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

[ 1 ]

Question (iii)

(iii)

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

[ 1 ]

Question 2

[Maximum number: 3]

Question (a)

(a)

An alkene, a carboxylic acid and a ketone, all of similar volatility, are mixed together. The mixture is then analysed by gas chromatography.

The gas chromatogram produced is shown.

Figure for Question (a) — CAIE A-Level Chemistry A2

The separation of the three compounds depends on their relative solubilities in the liquid stationary phase. The liquid stationary phase is an alkane.

[ 3 ]

Question (i)

(i)

Complete the table to suggest which compound in the mixture is responsible for each peak J, K and L. Explain your answer by reference to the intermolecular forces of each compound.

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

A student calculates the areas underneath the three peaks in the chromatogram.

Table for Question (i) — CAIE A-Level Chemistry A2
[ 2 ]

Question (ii)

(ii)

The area underneath each peak is proportional to the mass of the respective compound.

Calculate the percentage by mass in the original mixture of the compound responsible for peak K.
\% of mixture responsible for peak K =

[ 1 ]
All question bank results loaded