Edexcel A Level Chemistry:复习笔记7.7.1 Mass Spectrometry

Using Mass Spectra

 

  • At AS level, you have used mass spectrometry to determine:
    • Relative atomic mass from isotopic abundance
    • Possible molecular formulae from molecular ion peaks
    • Organic structures from fragmentation patterns
  • High resolution mass spectrometry can be used to deduce / confirm molecular formulae
  • High resolution mass spectrometers provide data accurate to four, or sometimes more, decimal places
  • This means that a compounds molecular formula can sometimes be deduced by using atomic masses accurate to four decimal places

Table of selected accurate relative atomic masses

Element Hydrogen Carbon Nitrogen Oxygen
Symbol H C N O
Accurate Ar 1.0078 12.0000 14.0031 15.9949

Worked Example

A compound is found to have an Mr value of 58.0417.

Determine if the compound is:

  1. C4H10
  2. C2H6N2
  3. C3H6O

   Answer   

Element Hydrogen Carbon Nitrogen Oxygen
Symbol H C N O
Accurate Ar 1.0078 12.0000 14.0031 15.9949

 

  1. C4H10 = (4 x 12.0000) + (10 x 1.0078) = 58.0780
  2. C2H6N2 = (2 x 12.0000) + (6 x 1.0078) + (2 x 14.0031) = 58.0530
  3. C3H6O =(3 x 12.0000) + (6 x 1.0078) + (15.9949) = 58.0417
  • Therefore, the correct formula of the compound is C3H6O

Exam Tip

  • High resolution mass spectrometry can inform you about the molecular formula, it cannot tell you about the actual structure
    • In the worked example above, the final answer was C3H6O but you cannot say whether this is propanal or propanone
    • You would have to use the fragments on the mass spectrum for further detail that might help deduce more information about the structure
  • Sometimes you may calculate the molecular mass of a compound and it won't match the value given in the question exactly
    • In this case, you should:
      • Firstly, double check your calculation(s)
      • Secondly, choose the compound that is the closest

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