Relative Mass 1

Relative Mass 1

1. Mass of atoms compared to C-12

   • Relative Atomic mass
     • Ar
     • average mass of atom where 1 atom of C-12 is 12
   • Relative Isotopic mass
     • mass of atom of isotope when 1 C-12 is 12
   • relative molecular/formula mass
     • average mass of molecule when 1 C-12 is 12
     • Mr

2. Can be measured using mass spectrometer

   • 1- vapourisation
     • sample turned into gas
   • 2- ionisation
     • gas particles bombarded with electrons
       • particle electrons knocked off- + ions
   • 3- Acceleration
     • + ions accelerated by electric field
   • 4 - detection
     • time taken for + ions to reach detector measured
       • depends on mass/charge
       • sample produces mass spectrum

3. Ar and Relative Isotopic Abundance can be worked out from a mass spectrum

   • 1- read % abundance from spectrum and relative isotopic mass from x axis
     • multiply to get total mass for each isotope
   • 2- add up totals
   • divide by 100
   • if abundance is not given as percentage...
     • 1- do steps 1 and 2 normally
     • 2- divide by total relative abundance instead of 100

4. Mass spectrometry also used to find Mr

   • molecular ion (M+) formed when bombarding electrons remove electron from molecule
     • gives peak in spectrum with highest mass
       • mass of M+ gives Mr for the molecule
   • electron bombardment also makes fragments of molecules
     • all show up on spectrum- fragmentation pattern
       • can be used to identify molecules and structures

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