Relative Masses

Relative masses are masses of atoms compared to carbon-12
1. mass of an atom is very tiny
2. mass of one atom is compared to the mass of a different atom- this is its relative mass
3. Relative atomic mass- average mass of an atom of an element
4. relative atomic mass- mass of an atom of an isotope of an element
5. relative molecular mass- average mass of a molecule or formula unit
Relative masses can be measured using a mass spectrometer
1. tells you the relative atomic mass, relative molecular mass, relative isotopic abundance, molecular structure
2. step 1- Vaporistation
  1. the sample is turned into gas using an electrical heater
3. step 2- Ionisation
  1. the gas particles are bombarded with high energy electrons to ionise them. Electrons are knocked off the particles leaving positive ions.
4. step 3- Acceleration
  1. the positive ions are accelerated by an electric field.
5. step 4- Deflection
  1. the positive ions paths are alternated with a magnetic field- lighter ions have less momentum and are deflected more than heavier ions- for a given magnetic field ions with a particular mass/charge ratio make it to the detector
6. step 5- Detection
  1. the magnetic field strength is slowly increased- as this happens different ions can reach the detector- a mass spectrum is produced.
Relative atomic mass and relative isotopic abudance can be worked out from a mass spectrum
1. step1
  1. for each peak read the % relative isotopic abundance and relative isotopic mass- multiply them together to get the total mass for each isotope
2. step 2
  1. add up all the totals
3. step 3
  1. divide answer by 100
    1. if the relative abundance is not given as a % divide by the total relative abundance instead of 100
Mass spectrometry can be used to find the relative molecular mass

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Relative Masses

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