Mass spectrometryEdexcel International A Level Chemistry: Subtopic test

1. 1
   A forensic laboratory analyses a metal sample using a mass spectrometer. The sample gives two peaks: one at m/z 63 with a relative abundance of 69.2 % and one at m/z 65 with a relative abundance of 30.8 %. All the ions have a charge of 1+.

   (a)

   What is the relative atomic mass of the metal, to three significant figures?

   [1 mark]

   • A63.0
   • B64.0
   • C63.6
   • D65.0

   (b)

   In the mass spectrometer, which process converts gaseous atoms of the metal into positive ions?

   [1 mark]

   • AIonisation by electron bombardment
   • BAcceleration by an electric field
   • CDeflection in a magnetic field
   • DDetection at a charged plate

   (c)

   Explain why the inside of a mass spectrometer is kept under a high vacuum.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Boron has two isotopes, ¹⁰B and ¹¹B. A mass spectrum of a boron sample gives a relative atomic mass of 10.8.

   (a)

   What is the percentage abundance of ¹⁰B in the sample?

   [1 mark]

   • A10 %
   • B80 %
   • C90 %
   • D20 %

   (b)

   At which m/z value would a ¹¹B²⁺ ion be detected?

   [1 mark]

   • A11
   • B5.5
   • C22
   • D9

   (c)

   A second sample of boron contains a greater proportion of ¹⁰B than the first. State and explain how its relative atomic mass compares with 10.8.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Chlorine consists of two isotopes, ³⁵Cl (75.0 %) and ³⁷Cl (25.0 %). A sample of chlorine gas, Cl₂, is analysed in a mass spectrometer, and molecular ions, Cl₂⁺, are detected.

   (a)

   Identify the molecular ions responsible for the peaks at m/z 70, 72 and 74.

   [3 marks]

   (b)

   Calculate the relative heights of the three peaks, showing your working.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student records the mass spectrum of a gaseous sample of an unknown element Y. The spectrum has three large peaks at m/z 24, 25 and 26 with relative heights of 79, 10 and 11, and three much smaller peaks at m/z 12, 12.5 and 13.

   (a)

   Describe how a mass spectrometer produces a mass spectrum of a gaseous sample.

   [6 marks]

   (b)

   Identify the ions responsible for the three large peaks, calculate the relative atomic mass of Y to three significant figures and name the element. Explain the origin of the three smaller peaks.

   [6 marks]

   Total for question 4: 12 marks

End of questions