Chemistry- Using a mass spectra

Calculating the relative atomic mass

You need to know how to calculate the relative atomic mass (Ar) Of an element from a mass spectrum.

Step 1: For each peak, read the % relative isotopic abundance from the Y axis. Multiply them together together the total relative mass for each isotope.

Step 2: add up these totals

Step 3: Divide by 100 ( since % were used)

Example

Here is how to calculate Ar for magnesium using the mass spectrum above.

Step 1:

Total Mass of 1st isotope: 79 × 24=1,896

Total Mass of 2nd isotope: 10 × 25=250

Total mass of 3rd isotope: 11 × 26=286

Step 2:

1896 + 250 + 286 =2,432

Step 3

2432 divided by 100 = 24.32

So Ar of Mg = 24.3 to 3 s.f

if the relative abundance is not given as a percentage, the total abundance may not add up to 100

If the relative abundance is not given as a percentage, the total abundance may not add up to 100. You do step one and two as normal, but you would divide by the total relative abundance instead of 100

Calculating relative molecular mass

You can also get a mass spectrum for a molecular sample. A molecular ion Mg+ Is formed when one electron is removed from the molecule.

This gives a peak in the spectrum with a mass/charge ratio equal to the Mr Of the molecule. This can be used to help identify an unknown compound.