Mass spectrometry and relative atomic massAQA A-Level Chemistry: Subtopic test

1. 1
   A chemist analyses a sample of copper in a time of flight (TOF) mass spectrometer. The sample is vaporised and ionised by electron impact to form ions with a 1+ charge. The ions are accelerated by an electric field, drift along a flight tube and then reach a detector. Copper has two isotopes, ⁶³Cu and ⁶⁵Cu.

   (a)

   In the TOF mass spectrometer, ⁶³Cu⁺ ions and ⁶⁵Cu⁺ ions are accelerated to the same kinetic energy. Which statement about their time of flight along the flight tube is correct?

   [1 mark]

   • ABoth ions reach the detector at the same time because they have the same charge
   • B⁶⁵Cu⁺ ions reach the detector first because they have more momentum
   • C⁶⁵Cu⁺ ions reach the detector first because they travel faster
   • D⁶³Cu⁺ ions reach the detector first because they travel faster

   (b)

   Which equation represents the ionisation of a gaseous copper atom by electron impact?

   [1 mark]

   • ACu(g) + e⁻ → Cu⁺(g) + 2e⁻
   • BCu(g) + e⁻ → Cu⁻(g)
   • CCu(s) → Cu⁺(g) + e⁻
   • DCu⁺(g) + e⁻ → Cu(g)

   (c)

   Explain why the ions must be accelerated to the same kinetic energy in a TOF mass spectrometer.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A sample of neon gas from the air is analysed in a mass spectrometer. The sample contains three isotopes, with these relative abundances: ²⁰Ne 90.5%, ²¹Ne 0.3% and ²²Ne 9.2%.

   (a)

   What is the relative atomic mass of this neon sample, to 3 significant figures?

   [1 mark]

   • A20.0
   • B21.0
   • C20.2
   • D22.0

   (b)

   In the mass spectrum of this sample, what does the height of each peak represent?

   [1 mark]

   • AThe speed of the ions
   • BThe relative abundance of that isotope
   • CThe number of neutrons in the isotope
   • DThe charge on the ions

   (c)

   A second sample of neon is enriched in the lighter isotope. It contains only ²⁰Ne and ²²Ne, with relative abundances of 80.0% and 20.0%. Calculate the relative atomic mass of this sample to 3 significant figures.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A mass spectrometer is used to analyse a sample of an unknown metal. The ions detected are all 1+ ions. The mass-to-charge ratios (m/z) of the peaks and their relative abundances are: m/z 54, 5.8%; m/z 56, 91.8%; m/z 57, 2.1%; m/z 58, 0.3%.

   (a)

   Calculate the relative atomic mass of the metal to 1 decimal place, and use the periodic table to identify the element.

   [3 marks]

   (b)

   The ⁵⁴Fe⁺ ions are detected before the ⁵⁶Fe⁺ ions. Explain why, and explain how the detector provides the data needed to find the relative abundance of each isotope.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A trainee analyst is learning how a TOF mass spectrometer is used to analyse a sample of chlorine. The spectrum obtained has two peaks, one at m/z 35 and one at m/z 37, with relative abundances of 75.8% and 24.2% respectively. All the ions have a 1+ charge.

   (a)

   Describe how the TOF mass spectrometer produces a spectrum of the chlorine sample, in the order that the sample passes through the instrument. Include an equation for the ionisation of a chlorine atom by electron impact.

   [6 marks]

   (b)

   Use the data to calculate the relative atomic mass of chlorine to 3 significant figures. Explain why this value is not a whole number, and explain how a mass spectrum can be used to identify an element.

   [6 marks]

   Total for question 4: 12 marks

End of questions