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