Relative formula mass
Lesson details
Learning outcome
I can calculate the relative formula mass of substances using data taken from a periodic table.
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.
Teacher tip
Model many worked examples, especially with compounds containing multiple atoms of particular elements.
Licence
except where otherwise stated. See Oak's terms & conditions (Collection 2).Lesson video
Loading...
Prior knowledge starter quiz
6 Questions
Q1.Which statement accurately describes the atomic number of any given element?
Q2.True or false: The atomic mass number of an element is listed on the periodic table.
Q3.Match each term with its correct definition.
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
Q4.Isotopes of the same element have the same atomic number because they contain the same number of protons. However, they have different relative atomic masses due to varying numbers of .
Q5.If an element has an atomic number of 8 and an average atomic mass of 16, which element is it?
Q6.The number is always a whole number because it represents the fixed number of protons in the nucleus of an atom.
Assessment exit quiz
6 Questions
Q1.What does the relative atomic mass (RAM or Ar) compare the mean mass of an atom to?
Q2.Order the following steps to calculate the RFM of a compound correctly. Start withe periodic table.
Q3.What is the relative formula mass (RFM or Mr) of water (H₂O)?
Q4.Calculate the RFM of carbon dioxide (CO₂)
Q5.To calculate the percentage by mass of an element in a compound, you need:
Q6.What is the percentage by mass of hydrogen in water (H₂O)? Give your answer to four significant figures.
To help you plan your 10 chemistry lesson on: Relative formula mass, download all teaching resources for free and adapt to suit your pupils' needs...
To help you plan your 10 chemistry lesson on: Relative formula mass, download all teaching resources for free and adapt to suit your pupils' needs.
The starter quiz will activate and check your pupils' prior knowledge, with versions available both with and without answers in PDF format.
We use learning cycles to break down learning into key concepts or ideas linked to the learning outcome. Each learning cycle features explanations with checks for understanding and practice tasks with feedback. All of this is found in our slide decks, ready for you to download and edit. The practice tasks are also available as printable worksheets and some lessons have additional materials with extra material you might need for teaching the lesson.
The assessment exit quiz will test your pupils' understanding of the key learning points.
Our video is a tool for planning, showing how other teachers might teach the lesson, offering helpful tips, modelled explanations and inspiration for your own delivery in the classroom. Plus, you can set it as homework or revision for pupils and keep their learning on track by sharing an online pupil version of this lesson.
Explore more key stage 4 chemistry lessons from the Atomic structure and the periodic table unit, dive into the full secondary chemistry curriculum, or learn more about lesson planning.