Relative mass

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  • Relative mass
    • The mass of a carbon-12 isotope is defined as exactly 12 atomic mass units, 12u
      • 1u= 1/12th the mass of a carbon 12 atom
        • Approx the mass of a proton or neutron
    • Relative isotopic mass is the mass of an isotope relative to 1/12th of the mass of an atom of carbon-12
      • Most elements contain a mixture of isotopes, each with a different isotopic mass
    • Relative atomic mass, Ar, is the weighted mean mass of an atom of an element relative to 1/12th of an atom of carbon-12
      • Weighted mean mass takes account of- percentage abundance of each isotope and relative isotopic mass of each isotope
        • Found using a mass spectrometer
          • A sample is vaporised and ionised to form positive ions, the ions are accelerated and heavier ions move slower and are harder to deflect so they separate. Ions are detected as m/z
          • m/z= mass to charge ration= relative mass of ion divided by relative charge of ion
            • A sample is vaporised and ionised to form positive ions, the ions are accelerated and heavier ions move slower and are harder to deflect so they separate. Ions are detected as m/z
      • To find Ar of an isotope. Ar= (%iso1xMassnoiso1)+(%iso2xMassnoiso2) divided by 100 or total abundance

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