High-resolution mass spectrometry and chromatographyEdexcel International A Level Chemistry: Subtopic test

1. 1
   Carbon monoxide, nitrogen and ethene all have the same nominal relative molecular mass of 28. A chemist uses a high-resolution mass spectrometer to analyse gases. Accurate relative atomic masses: H = 1.0078, C = 12.0000, N = 14.0031, O = 15.9949.

   (a)

   Why can a high-resolution mass spectrometer distinguish between carbon monoxide, nitrogen and ethene?

   [1 mark]

   • AThe three compounds have different numbers of electrons, which changes their peak heights
   • BThe compounds have different nominal relative molecular masses
   • CThe accurate relative atomic masses of the isotopes are not whole numbers, so the three molecular ions have slightly different accurate masses
   • DThe molecular ions have different charges

   (b)

   A molecular ion peak is recorded at m/z 28.0312. Which compound gives this peak?

   [1 mark]

   • ACO
   • BN₂
   • CHCN
   • DC₂H₄

   (c)

   Calculate the accurate relative molecular mass of propanone, C₃H₆O, to four decimal places.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student uses one-way paper chromatography to separate a mixture of two coloured food dyes, X and Y, using water as the solvent. The solvent front moves 8.0 cm from the pencil start line. The centre of the spot for dye X is 3.2 cm from the start line and the centre of the spot for dye Y is 5.6 cm from the start line.

   (a)

   What is the stationary phase in paper chromatography?

   [1 mark]

   • AThe water used as the solvent
   • BWater bound to the cellulose fibres of the paper
   • CThe pencil line
   • DThe air above the paper

   (b)

   What is the Rf value of dye Y?

   [1 mark]

   • A0.70
   • B0.56
   • C0.40
   • D1.43

   (c)

   Explain why dye Y travels further up the paper than dye X.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A sports laboratory tests an athlete's urine sample for a banned stimulant using gas chromatography combined with mass spectrometry (GC-MS). A peak with retention time 4.2 minutes matches the retention time of a standard sample of the stimulant, which has the formula C₁₀H₁₅N. The mass spectrometer gives a molecular ion peak at m/z 149.1201 for this peak. Accurate relative atomic masses: H = 1.0078, C = 12.0000, N = 14.0031, O = 15.9949.

   (a)

   Explain why combining gas chromatography with mass spectrometry gives a more reliable identification of the stimulant than gas chromatography alone.

   [3 marks]

   (b)

   Show that the molecular ion peak is consistent with C₁₀H₁₅N and not with C₉H₁₁NO, by calculating the accurate relative molecular mass of each.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist monitors the formation of an ester from a carboxylic acid by thin-layer chromatography on a silica gel plate, using a non-polar solvent. The solvent front is 7.5 cm above the start line. A pure sample of the acid runs to 1.2 cm above the start line and a pure sample of the ester runs to 5.4 cm. A sample of the reaction mixture gives two spots, at 1.2 cm and 5.4 cm. The chemist then plans to check the purity of the product using gas chromatography or high-performance liquid chromatography, each combined with mass spectrometry.

   (a)

   Calculate the Rf value of each spot, deduce what the chromatogram shows about the reaction mixture, and explain why the ester has a higher Rf value than the acid.

   [6 marks]

   (b)

   Compare how gas chromatography and high-performance liquid chromatography separate the components of a mixture, and explain how combining either with mass spectrometry identifies the components.

   [6 marks]

   Total for question 4: 12 marks

End of questions