Relative masses and mass spectrometryEdexcel A-Level Chemistry: Subtopic test

1. 1
   A sample of magnesium was analysed by mass spectrometry. It contains three isotopes: ²⁴Mg (78.6%), ²⁵Mg (10.1%) and ²⁶Mg (11.3%).

   (a)

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

   [1 mark]

   • A24.0
   • B25.0
   • C24.3
   • D2433

   (b)

   The relative isotopic mass of ²⁵Mg is measured on a scale based on which standard?

   [1 mark]

   • A1/12 of the mass of an atom of carbon-12
   • BThe mass of an atom of hydrogen-1
   • C1/16 of the mass of an atom of oxygen-16
   • DThe mass of one mole of carbon-12 atoms in grams

   (c)

   Explain why the relative atomic mass of magnesium is not a whole number, even though the mass number of each isotope is a whole number.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Chlorine consists of two isotopes, ³⁵Cl and ³⁷Cl, in an abundance ratio of 3 : 1. A sample of chlorine gas, Cl₂, is analysed in a mass spectrometer in which each molecule is ionised to form a Cl₂⁺ ion.

   (a)

   How many different Cl₂⁺ molecular ion peaks appear in the mass spectrum of chlorine?

   [1 mark]

   • AOne, because both isotopes are chlorine
   • BTwo, one for each isotope
   • CFour, one for each possible pair of atoms in a molecule
   • DThree, at m/z 70, 72 and 74

   (b)

   Which is the m/z value of the tallest peak in the Cl₂⁺ molecular ion region?

   [1 mark]

   • A35
   • B70
   • C72
   • D74

   (c)

   Predict the relative heights of the peaks in the Cl₂⁺ region of the mass spectrum of chlorine. Show your working.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A sample of copper from a coin was analysed by mass spectrometry. The spectrum of Cu⁺ ions has two peaks: m/z 63 with relative abundance 69.1 and m/z 65 with relative abundance 30.9. A second sample of copper, from a meteorite, has a relative atomic mass of 63.75.

   (a)

   Calculate the relative atomic mass of the copper from the coin. Give your answer to 3 significant figures.

   [3 marks]

   (b)

   Calculate the percentage abundance of ⁶⁵Cu in the meteorite sample.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist uses mass spectrometry to analyse two substances. The peak with the highest m/z in the spectrum of propanone, (CH₃)₂CO, is at m/z 58. Bromine, Br₂, consists of the isotopes ⁷⁹Br and ⁸¹Br in approximately equal abundance. Relative atomic masses: H = 1.0, C = 12.0, O = 16.0.

   (a)

   Explain how the mass spectrum of propanone is used to determine its relative molecular mass, and show that the peak at m/z 58 is consistent with the formula (CH₃)₂CO.

   [6 marks]

   (b)

   Predict the molecular ion peaks in the mass spectrum of bromine. Give the m/z values and relative heights of the peaks, explaining how you decided on them.

   [6 marks]

   Total for question 4: 12 marks

End of questions