Use mass spectra and abundance data to calculate weighted atomic masses.
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Method and worked examples
Relative atomic mass is an isotope-weighted mean compared with one twelfth of a carbon−12 atom’s mass. It is dimensionless, whereas molar mass has units gmol−1. A non-integer mean does not describe an atom with a fraction of a neutron. It summarises a mixture of isotope masses.
Multiply each isotope mass by its fractional abundance, then add. Divide percentages by 100 first and check that fractions sum to one. Simplified questions may use mass numbers as approximate isotope masses. For equal instrument responses, normalise relative mass-spectrum peak signals by their total. A valid weighted mean lies between the smallest and largest isotope masses present.
Example 1
An element is 75.0% isotope mass 35 and 25.0% isotope mass 37. Find its relative atomic mass using these approximate masses.
Ar=0.750(35)+0.250(37)=35.5.
Example 2
An element has isotopes of approximate masses 10 and 11, with mean 10.8. Find the percentage of mass−11 isotope.
Let x be its fraction: 10(1−x)+11x=10.8, so x=0.8, or 80%.
Watch out:It is an isotope-weighted average, not the mass number of one atom.
Isotope abundance and relative atomic mass practice | Year 11 Chemistry | Ascent Arcade