Relative Masses

Relative Masses

1. Relative masses are masses of atoms compared to carbon-12

   • mass of an atom is very tiny
   • mass of one atom is compared to the mass of a different atom- this is its relative mass
   • Relative atomic mass- average mass of an atom of an element
   • relative atomic mass- mass of an atom of an isotope of an element
   • relative molecular mass- average mass of a molecule or formula unit

2. Relative masses can be measured using a mass spectrometer

   • tells you the relative atomic mass, relative molecular mass, relative isotopic abundance, molecular structure
   • step 1- Vaporistation
     • the sample is turned into gas using an electrical heater
   • step 2- Ionisation
     • the gas particles are bombarded with high energy electrons to ionise them. Electrons are knocked off the particles leaving positive ions.
   • step 3- Acceleration
     • the positive ions are accelerated by an electric field.
   • step 4- Deflection
     • 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
   • step 5- Detection
     • the magnetic field strength is slowly increased- as this happens different ions can reach the detector- a mass spectrum is produced.

3. Relative atomic mass and relative isotopic abudance can be worked out from a mass spectrum

   • step1
     • for each peak read the % relative isotopic abundance and relative isotopic mass- multiply them together to get the total mass for each isotope
   • step 2
     • add up all the totals
   • step 3
     • divide answer by 100
       • if the relative abundance is not given as a % divide by the total relative abundance instead of 100

4. Mass spectrometry can be used to find the relative molecular mass