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.
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.
What can you explain now?
Using C=12 and O=16, find Mᵣ of CO₂.
Compare with the explanation
44
There is one carbon and two oxygen atoms: 12+2×16=44.
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