Station 4 - Mass Spectrometry
1. When a sample of an unknown element is vaporized and injected into a mass spectrometer, the results shown below are obtained. Use these data to estimate the average atomic mass of this element:

(a)117 amu
(b)Between 117 and 118 amu
(c)Between 118 and 119 amu
(d)Between 119 and 120 amu
2. An unnamed element with an atomic number of 130 is vaporized and injected into a mass spectrometer. The results are shown below. Use these data to calculate the average atomic mass of this element.

(a)320 amu
(b)321 amu
(c)322 amu
(d)324 amu
3. Naturally occurring magnesium has the following isotopic abundances:
Isotope / Abundance (%) / Atomic Mass (amu)
24Mg / 78.99 / 23.98504
25Mg / 10.00 / 24.98584
26Mg / 11.01 / 25.98259
(a)What is the average atomic mass of Mg?
(b)Sketch the mass spectrum of Mg.
  1. (a) Thomson’s cathode-ray tube and the mass spectrometer both involve the use of electric or magnetic fields to deflect charged particles. What are the charged particles involved in each of these experiments?
(b) What are the labels on the axes of a mass spectrum? (
c) To measure the mass spectrum of an atom, the atom must first lose on or more electrons. Which would you expect to be deflected more by the same setting of the electric and magnetic field, a Cl+ or a Cl2+ ion?
  1. (a) The mass spectrometer has a magnet as one of its components. What is the purpose of the magnet?
(b) The atomic weight of Cl is 35.5 amu. However, the mass spectrum of Cl does not show a peak at this mass. Explain.
(c) A mass spectrum of phosphorus (P) atoms shows only a single peak at a mass of 31. What can you conclude from this observation?
  1. 6. The mass spectrum of Ch3Cl is illustrated here. You know that carbon has two stable isotopes, 12C and 13C with relative abundances of 98.9% and 1.1% respectively, and chlorine has two isotopes, 35Cl and 37Cl with abundances of 75.77% and 24.23%, respectively.
(a)What molecular species gives rise to the lines at m/Z of 50 and 52 about 1/3 the height of the line at 50?
(b)What species might be responsible for the line at m/Z = 51?

7. Mass spectrometric analysis showed that there are four isotopes of an unknown elements having the following masses and abundances:
Isotope / Mass Number / Isotope Mass / Abundance (%)
1 / 136 / 135.9090 / 0.193
2 / 138 / 137.9057 / 0.25
3 / 140 / 139.9053 / 88.48
4 / 142 / 141.9090 / 11.07
Three elements in the periodic table that have atomic weights near these values are: (a) lanthanum (La), atomic number 57, atomic weight 138.9055
(b) cerium (Ce), atomic number 58, atomic weight 140.115
(c)Praseodymium (Pr), atomic number 59, atomic weight 140.9076
Using the Data above, calculate the atomic weight and identify the element