Relative atomic, molecular and formula masses
Use relative masses consistently and count every atom before adding contributions.
Section 3.2 · Relative masses of atoms and molecules
Core and Extended · Chemistry 0620, 2026–2028.
Before you begin
- Read formulas and bracket multipliers.
- Know that isotopes have different neutron counts but the same element identity.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many oxygen atoms are counted in Ca(NO₃)₂?
What you will learn
- Explain relative atomic mass and distinguish it from isotope mass number.
- Calculate relative molecular or formula mass, including brackets.
A relative value has a reference, not a gram unit
Relative atomic mass Aᵣ is the mean mass of an element's atoms compared with one twelfth of the mass of a carbon-12 atom. It reflects the element's isotopes and their abundances. Because it is a ratio of masses, it has no unit. Aᵣ can be a decimal, unlike the whole nucleon count of a particular isotope's mass number.
Use the Aᵣ values supplied in the question or its Periodic Table. Chlorine may be given as 35.5; that is not an instruction to give a chlorine isotope half a neutron. Calculating an abundance-weighted Aᵣ is taught in the Extended isotope lesson; here, use the stated relative values correctly.
Add one contribution for every atom
Relative molecular mass Mᵣ is the sum of the relative atomic masses of the atoms in a molecule. For an ionic substance, call the corresponding sum relative formula mass because the formula describes an ion ratio rather than an isolated molecule. Count the atoms first, multiply by each Aᵣ and then add.
For Al₂(SO₄)₃, there are two aluminium, three sulfur and twelve oxygen atoms. Using Al=27, S=32 and O=16 gives 2×27+3×32+12×16=342. A coefficient is not part of the substance's Mᵣ: 2H₂O represents two water molecules, but water's relative molecular mass remains 18 using H=1 and O=16.
Pause and explain
Which statement about a relative formula mass of 80 is correct?
Worked example
Calculate the relative formula mass of Ca(NO₃)₂ using Ca=40, N=14 and O=16.
Show the worked solution
- Count one calcium, two nitrogen and six oxygen atoms.
- Add 40+2×14+6×16.
- The total is 164; keep the relative result without g or mol units.
Answer Mᵣ =164.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Find Mᵣ of Mg(OH)₂ using Mg=24, O=16 and H=1.
Give me a hint
Count two complete hydroxide groups.
Compare my reasoning
- The counts are one Mg, two O and two H.
- Calculate 24+2×16+2×1.
- The relative formula mass is 58 with no unit.
58
Look for these in your work
- Both oxygen and hydrogen receive the multiplier.
- The answer is distinguished from a gram sample mass.
Find Mᵣ of Al₂(SO₄)₃ using Al=27, S=32 and O=16, showing your atom counts.
Give me a hint
Multiply each atom inside the sulfate bracket by three.
Compare my reasoning
- Counts are 2 Al, 3 S and 12 O.
- Contributions are 54, 96 and 192.
- Their sum is 342.
342
Look for these in your work
- All formula-unit counts are explicit.
- The contributions are added without including an equation coefficient.
A student gives Mᵣ of water as 36 because an equation contains 2H₂O. Explain what is doubled and give water's Mᵣ using H=1 and O=16.
Give me a hint
A coefficient counts whole molecules; Mᵣ describes the substance's formula.
Compare my reasoning
- One H₂O has relative molecular mass 2×1+16=18.
- The coefficient two doubles the number of water molecules represented.
- It does not change water's molecular formula or its Mᵣ.
Water's Mᵣ remains 18; the coefficient doubles particle count.
Look for these in your work
- Molecular identity is kept fixed.
- Coefficient and subscript roles are distinguished.
Common mistakes
- Attaching grams to a relative mass.
- Multiplying only oxygen, rather than all atoms inside a bracket.
What can you explain now?
Find Mᵣ of NH₄NO₃ using N=14, H=1 and O=16.
Compare with the explanation
80
There are two nitrogens, four hydrogens and three oxygens: 2×14+4×1+3×16=80.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Calculate two bracketed relative formula masses without looking at a worked example. Explain why the answer has no unit and why an equation coefficient is excluded.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.