1. 1
   Selenium-79 is a long-lived radioactive isotope found in spent nuclear fuel. In the environment it can occur as the selenide ion, $^{79}_{34}\mathrm{Se}^{2-}$.

   (a)

   How many electrons are in this ion?

   [1 mark]

   • A32
   • B34
   • C45
   • D36

   (b)

   How many neutrons are in this ion?

   [1 mark]

   • A34
   • B45
   • C43
   • D79

   (c)

   Identify which one of the following atoms has the same number of neutrons as selenium-79, showing your reasoning: $^{80}_{35}\mathrm{Br}$, $^{80}_{36}\mathrm{Kr}$, $^{78}_{34}\mathrm{Se}$.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A sample of magnesium is analysed in a mass spectrometer. The spectrum shows three main peaks at m/z = 24, 25 and 26 with relative abundances of 78.99 %, 10.00 % and 11.01 % respectively. Three much smaller peaks appear at m/z = 12, 12.5 and 13.

   (a)

   What do the three main peaks show about the magnesium sample?

   [1 mark]

   • AMagnesium atoms have three different atomic numbers
   • BMagnesium forms ions with charges of 1+, 2+ and 3+
   • CMagnesium has three isotopes, and magnesium-24 is the most abundant
   • DThe sample is contaminated with two other elements

   (b)

   What is the relative atomic mass of this magnesium, to two decimal places?

   [1 mark]

   • A24.32
   • B24.00
   • C24.67
   • D25.00

   (c)

   Explain the origin of the small peak at m/z = 12.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The mass spectrum of a sample of bromine vapour shows two peaks from single bromine atom ions at m/z = 79 and 81, with relative abundances of 50.7 % and 49.3 %. It also shows three peaks from Br₂⁺ molecular ions at m/z = 158, 160 and 162, whose heights are in the ratio of approximately 1 : 2 : 1. The IB data booklet gives the relative atomic mass of bromine as 79.90.

   (a)

   Calculate the relative atomic mass of bromine from the atomic ion peaks, to two decimal places, and suggest why your answer differs slightly from the data booklet value.

   [3 marks]

   (b)

   Explain the m/z values of the three molecular ion peaks and the 1 : 2 : 1 ratio of their heights.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Archaeologists want to find out where the lead in an ancient water pipe was mined. Lead (Z = 82) has four stable isotopes. Mass spectra, recorded as 1+ ions, give the following relative abundances at m/z = 204, 206, 207 and 208. Pipe: 1.4 %, 24.1 %, 22.1 % and 52.4 %. Ore from mine P: 1.4 %, 24.0 %, 22.2 % and 52.4 %. Ore from mine Q: 1.2 %, 26.0 %, 19.6 % and 53.2 %.

   (a)

   Determine the relative atomic mass of lead in the pipe and in each ore, to two decimal places, using the m/z values as isotopic masses. Deduce the number of neutrons in the most abundant isotope, and comment on what your relative atomic masses show.

   [6 marks]

   (b)

   Evaluate, using the full isotopic data, which mine is the more likely source of the lead in the pipe, and discuss the limitations of this conclusion.

   [6 marks]

   Total for question 4: 12 marks

End of questions