IGCSE · Cambridge (CIE) · 0971

Relative formula mass

Read a chemical formula as an exact count of atoms before adding their masses.

What you will learn

Count atoms using subscripts and brackets.
Calculate relative formula mass.

A formula counts particles

A subscript applies to the element immediately before it. A number after a bracket multiplies every atom inside that bracket. An omitted subscript means one atom. A coefficient in a reaction equation counts formula units rather than changing the formula of the substance.

Add the relative atomic masses

Relative formula mass is the sum of relative atomic masses for all atoms in the formula. It is a ratio and has no unit. Use the atomic masses supplied in the question, which may be rounded differently from other tables.

For ionic compounds such as magnesium oxide, 'formula mass' is more suitable than suggesting a separate molecule. The formula gives the simplest ratio of ions in the lattice.

Mᵣ = sum of all Aᵣ values in the formula

Put the idea to work

Worked example

Using Ca=40, O=16 and H=1, find Mᵣ of Ca(OH)₂.

Show the worked solution

Count one calcium, two oxygen and two hydrogen atoms.
Calculate 40+2×16+2×1.
Add without attaching g or mol units to the relative value.

Answer 74

Common mistakes

Do not apply a bracket multiplier to only the last element.
Relative formula mass and mass in grams are different quantities.

Recall without your notes

What can you explain now?

Using C=12 and O=16, find Mᵣ of CO₂.

Compare with the explanation

There is one carbon and two oxygen atoms: 12+2×16=44.

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Keep learning Chemistry (9-1)

Check the official syllabus 2026–2028

Course scope and assessment

143 sequenced lessons cover all twelve syllabus areas with 429 guided, independent and application problems. 49 of the 49 syllabus sections have linked teaching and practice. This measures availability of mapped content, not full programme completion or guaranteed exam readiness. Three earlier starter lessons remain available. Extend your formula notebook as new reagents and ions appear; the starter notebook is not a universal formula reference. Paper-based method exercises support laboratory learning but do not demonstrate practical competence or complete either practical-assessment route. Written practice is self-checked. This is the separate 9–1 qualification 0971; Cambridge states that its 2026–2028 subject content is otherwise the same as 0620. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. The 2029 syllabus is separately published at https://www.cambridgeinternational.org/Images/764365-2029-syllabus.pdf and is not yet mapped here. This course supports exams in 2026, 2027 and 2028 only.