High Resolution Mass Spectrometry (Edexcel International A Level Chemistry): Revision Note

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

Determine if the compound is:

1. C₄H₁₀ 

2. C₂H₆N₂ 

3. C₃H₆O

 Answer   

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 

• 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