Relative Formula Mass
Relative formula mass is the sum of the relative atomic masses of all the atoms in a formula.
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Relative formula mass is the sum of the relative atomic masses of all the atoms in a formula.
- For CO2 add 12 and 2 x 16 to get 44
- It has no units
- Used to calculate moles
🧠 Quick quiz
Three questions. Score 2 or more to complete the topic.
What is Relative Formula Mass?
Within Chemistry, which covers atoms, bonding, reactions, the periodic table, organic chemistry and the chemistry of everyday life, this topic explains Relative Formula Mass. Chemistry links what you can see in the lab to the particles and bonds behind it, so each topic here shows that link in a model.
Check that you can explain Relative Formula Mass in your own words and give one example from everyday life. If you can do that, you have understood the core idea.
Understanding Relative Formula Mass helps you explain why substances behave as they do, from cleaning products and food to medicines.
Relative Formula Mass in detail
The relative formula mass (Mᵣ) is the sum of the relative atomic masses (Aᵣ) of all the atoms in the formula. For water, H₂O, Mᵣ = (2 × 1) + 16 = 18, and for calcium carbonate, CaCO₃, Mᵣ = 40 + 12 + (3 × 16) = 100.
It has no units, as it is relative to carbon-12. Mᵣ lets you compare the masses of different substances and is needed to calculate moles and the percentage by mass of an element in a compound. Use the Aᵣ values from the periodic table and take care to multiply by any subscripts or brackets.
How the interactive helps you understand Relative Formula Mass
Watch the bar chart in the interactive. Use any sliders or buttons it offers to change the values and see how the bar chart responds.
Before you change anything, predict the result. Then change one value at a time and check whether your prediction was right. This is a quick way to build a correct mental model of Relative Formula Mass.
Real-life uses and examples
- Use Relative Formula Mass to explain what happens in a lab experiment and why.
- Link Relative Formula Mass to everyday products such as cleaning agents, food and medicines.
- Read a chemical equation or diagram and work out what Relative Formula Mass shows.
Common mistakes to avoid
- Describing what happens without explaining the particles or bonds involved.
- Mixing up the names of substances or ions when you describe Relative Formula Mass.
- Learning the definition without testing it. Use the quiz and the interactive to check that you can apply the idea.
Frequently asked questions about Relative Formula Mass
What is Relative Formula Mass?
Relative formula mass is the sum of the relative atomic masses of all the atoms in a formula.
What are the most important points about Relative Formula Mass?
The main points are: For CO2 add 12 and 2 x 16 to get 44; It has no units; Used to calculate moles.
Why is Relative Formula Mass useful?
Understanding Relative Formula Mass helps you explain why substances behave as they do, from cleaning products and food to medicines.
How can I practise Relative Formula Mass?
Open the interactive at the top of this page and change the controls one at a time. Predict the result before each change, then take the three-question quiz. A score of two or more marks the topic as complete on this device.
Which topics are related to Relative Formula Mass?
Related topics in Chemistry include Percentage Yield and Atom Economy, Acids and Alkalis, Neutralisation.