Topic 7: Modern Analytical Techniques I

Question 1

How do you find the relative molecular mass of a compound using a mass spectrum?

Answer 1

• look at the molecular ion peak (the M peak)
• it has the highest m/z value
• that is the molecular mass

Question 2

How can the molecular ion be broken down?

Answer 2

• the bombarding electrons make some of the molecular ions break up into fragments
• these show up, making a fragmentation pattern
• you can use this to identify molecules and their structure

Question 3

How do you work out the structural formula using a mass spectrum?

Answer 3

• identify the fragments
• piece them together to form a molecule with the correct molecular mass

Question 4

How can mass spectrometry be used to differentiate between similar molecules?

Answer 4

• even if they contain the same atoms, they won’t produce the same set of fragments

Question 5

How does infrared spectroscopy work?

Answer 5

• a beam of IR radiation is passed through a sample of a chemical
• the IR radiation is absorbed by the covalent bonds in the molecules, increasing their vibrational energy
• bonds between different atoms absorb different frequencies of IR radiation
• bonds in different places in a molecule absorb different frequencies too

Question 6

How can you show if a reaction has happened using IR spectroscopy?

Answer 6

• it is good at telling if a functional group has changed during a reaction
• do some practise qs girlll