Mass spectrometry in organic analysisAQA A-Level Chemistry: Subtopic test

1. 1
   A pure organic compound is analysed by mass spectrometry. In the mass spectrum, the peak with the highest mass-to-charge ratio (ignoring any very small peaks above it) is at m/z = 74.

   (a)

   What does the peak at m/z = 74 represent?

   [1 mark]

   • AThe molecular ion, which gives the relative molecular mass
   • BThe base peak, which is the fragment with the most atoms
   • CA fragment ion formed by loss of a hydrogen atom
   • DThe ion formed by the carbon-13 isotope only

   (b)

   Which compound could give this molecular ion peak?

   [1 mark]

   • APropan-2-ol, C₃H₈O
   • BButanal, C₄H₈O
   • CButan-2-ol, C₄H₁₀O
   • D2-Methylpropanal, C₄H₈O

   (c)

   The compound is known to contain only carbon, hydrogen and oxygen. Explain why the m/z value of 74 alone cannot be used to find its molecular formula.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   High-resolution mass spectrometry measures relative molecular masses to four decimal places. Precise relative atomic masses: H = 1.0078, C = 12.0000, N = 14.0031, O = 15.9949.

   (a)

   Why do compounds with the same whole-number relative molecular mass have different precise relative molecular masses?

   [1 mark]

   • AThey contain different numbers of electrons
   • BThe precise masses of atoms are not whole numbers, so different combinations of atoms give different totals
   • CThe molecular ion loses mass as it travels through the spectrometer
   • DThey have different numbers of neutrons in each molecule

   (b)

   What is the precise relative molecular mass of propan-1-ol, C₃H₈O?

   [1 mark]

   • A60.0000
   • B60.0210
   • C59.0495
   • D60.0573

   (c)

   A compound has a precise relative molecular mass of 46.0054. Calculate the precise relative molecular masses of C₂H₆O and CH₂O₂ and decide which is the molecular formula of the compound.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A pharmaceutical analyst finds that a compound containing only carbon, hydrogen and oxygen has a precise relative molecular mass of 88.0524 by high-resolution mass spectrometry. Precise relative atomic masses: H = 1.0078, C = 12.0000, O = 15.9949.

   (a)

   The compound has a nominal mass of 88. Calculate the precise relative molecular masses of C₄H₈O₂ and C₅H₁₂O and deduce the molecular formula of the compound.

   [3 marks]

   (b)

   The compound fizzes when added to aqueous sodium carbonate. Identify the functional group and give the names and structural formulae of two possible isomers. Explain why high-resolution mass spectrometry cannot tell these two isomers apart.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A forensic laboratory uses high-resolution mass spectrometry to identify unknown substances. Precise relative atomic masses: H = 1.0078, C = 12.0000, N = 14.0031, O = 15.9949.

   (a)

   A gas from a sealed container has a molecular ion peak at m/z = 28.0313. The gas could be carbon monoxide, nitrogen or ethene. Calculate the precise relative molecular mass of each, deduce the gas, and explain why a low-resolution spectrometer could not identify it.

   [6 marks]

   (b)

   A laboratory technician says that a molecular ion peak at m/z = 72 proves that a liquid is butanal, C₄H₈O. Evaluate this claim, using precise relative molecular masses of other possible formulae and the structure of the compound.

   [6 marks]

   Total for question 4: 12 marks

End of questions