Relative Mass

HideShow resource information
  • Created by: chunks-42
  • Created on: 21-04-15 17:11

Relative Masses are Masses of Atoms Compared to Ca

The actucal mass of an atom is very, very tiny. Don't worry about exactly how tiny for now, but it's far too small to weigh. So, the mass of one atom is compared to the mass of a different atom. This is its relative mass. Here are some definitions to learn:

The relative atomic mass is the average mass of an atom of an element on a scale where an atom of carbon-12 is 12.

The relative molecular mass is the average mass of a molecule of formula unit on a scale where an atom of carbon-12 is 12.

1 of 4

Relative Masses can be Measured Using a Mass Spect

You can use a mass spectrometer to find out loads of stuff. It can tell you the relative atomic mass, relative molecular mass, and molecular structure.

There are 5 things that happen when a sample is squirted into a mass spectrometer:

1. Vaporisation- the sample is turned into gas (vaporised) using an electrical heater.

2. Ionisation- the gas particles are bombared with high-energy electrons to ionise them. Electrons are knocked off the particles, leaving positive ions.

3. Acceleration- the positive ions are accelerated by an electric field.

4. Deflection- the positive ions are altered with a magnetic field. Lighter ions have less momentum and are deflected more than heavier ions. For a given magnetic field, only ions with a particular mass/charge ratio make it to the detector.

5. Detection- the magnetic field strength is slowly increased. As this happens, different ions (ones with a higher mass/charge ratio) can reach the detector. A mass spectrum is produced.

2 of 4

Ar can be Worked Out from a Mass Spectrum

You need to know how to calculate the relative atomic mass of an element from the mass spectrum.

Here's how to calculate Ar for magnesium, using the mass spectrum below-

1. For each peak, read the % relative isotopic abundance from the y axis and the relative isotopic mass from the x-axis. Multiply them together to get the total mass for each isotope.

     79x24=1896, 10x25=250, 11x26=286

2. Add up these totals: 1896+250+286= 2432

3. Divide by the total relative abundance: 2432/100= 24.32

3 of 4

Mass Spectrometry can be used to Find Out Mr

You can also get a mass spectrum for a maolecular sample, such as ethanol.

1. A molecular ion is formed when the bombarding electrons remove 1 electron from the molecule.This gives the peak in the spectrum with the highest mass (furthest to the right, ignoring isotopes). The mass of a molecular ion gives the Mr for the molecule. E.g. ethanol has Mr=46

2. But it's not that simple- bombarding with electrons makes some molecules break up into fragments. These all show up on the mass spectrum, making a fragmentation pattern.

e.g. for ethanol, the fragments you get include CH3 + (Mr=15), CH3CH2 + (Mr=29) and

CH2OH + (Mr=31)

Fragmentation patterns are actucally pretty cool because you can use them to identify molecules and even their structure.

4 of 4


No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Atomic Structure resources »