Relative formula mass
I can calculate the relative formula mass of substances using data taken from a periodic table.
Relative formula mass
I can calculate the relative formula mass of substances using data taken from a periodic table.
These resources will be removed by end of Summer Term 2025.
Lesson details
Key learning points
- The relative atomic mass is the mean mass of an atom compared to 1/12 of the mass of a carbon-12 atom.
- The relative formula mass (RFM or Mr) is the sum of the relative atomic masses of all the atoms in a chemical formula.
- %mass of an element in a compound = (relative mass of element in compound ÷ RFM of compound) × 100
Keywords
Relative atomic mass - RAM is the mean mass of an atom relative to 1/12th the mass of an atom of carbon–12.
Relative formula mass - (RFM or Mr) is the sum of the relative atomic masses of all the atoms in a chemical formula.
Common misconception
Pupils may use the atomic number rather than mass when calculating the relative formula mass. Also, the number of atoms can be missed for the percentage of an element in a compound.
Emphasise that atomic mass is the larger number for each element on the periodic table and encourage pupils to use the key present to help them identify the mass.
Licence
This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).
Lesson video
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Starter quiz
6 Questions
the number of protons in the nucleus of an atom
sum of the number of protons and neutrons in the nucleus of an atom
average mass of an element's isotopes, based on their abundances