CAIE A-Level Chemistry 37.2.2 Percentage Composition from Chromatograms

Practise converting gas–liquid chromatogram peak areas into the percentage composition of a mixture.

Exam points

measure or use the integrated area of each chromatogram peak rather than peak height alone
sum all relevant peak areas, then calculate component area ÷ total area × 100%
check that calculated component percentages total approximately 100%, allowing for rounding

[Maximum number: 1]

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

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.

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

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

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37.2.2—Gas / liquid chromatograms question 1