Mini Chemistry
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Relative Formula Mass and Molar Mass

Use atomic masses and bracket multipliers, then distinguish relative mass from mass per mole.

  • SEC G2 Science Chemistry component 2027
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Learning objectives

  • define relative atomic mass, Ar
  • define relative molecular mass, Mr, and calculate relative molecular mass (and relative formula mass) as the sum of relative atomic masses

Relative mass is a comparison; molar mass tells you a mass per mole. Their numerical values match when molar mass is expressed in g mol⁻¹, but their meanings and units differ.

Build a relative mass from the formula

Relative atomic mass, Aᵣ, compares the weighted mean mass of an atom with one-twelfth of the mass of a carbon-12 atom. It has no unit. Relative molecular mass, Mᵣ, adds the Aᵣ values for all atoms in a molecule. For an ionic substance, use relative formula mass, because there are no individual molecules of that ionic compound.

For CaCO₃, with Aᵣ(Ca) = 40, Aᵣ(C) = 12 and Aᵣ(O) = 16, count one calcium, one carbon and three oxygens:

Mᵣ = 40 + 12 + 3(16) = 100.

The molar mass is therefore 100 g mol⁻¹: one mole has mass 100 g. Writing “Mᵣ = 100 g” would confuse a relative value with a sample’s mass.

Keep every bracket multiplier

Examiner practice 1

Calculate a Bracketed Formula Mass

3 marks

Examination question

Calculate Mᵣ of Ca(OH)₂ using Aᵣ(Ca) = 40, Aᵣ(O) = 16 and Aᵣ(H) = 1. [3 marks]

Show a markable method

View solution step by step

  1. Interpret the bracket

    1 mark

    Method

    Count two oxygen and two hydrogen atoms.

    Reason

    The outside subscript 2 multiplies everything inside (OH).

    Working

    Ca₁O₂H₂

  2. Substitute relative masses

    1 mark

    Method

    Add every atomic contribution.

    Reason

    Mᵣ is the total of the represented relative atomic masses.

    Working

    40 + 2(16) + 2(1)

  3. Evaluate

    1 mark

    Method

    State the relative formula mass.

    Reason

    The arithmetic total is 74.

    Working

    Mᵣ = 74

Try a new formula

Use Aᵣ(Mg) = 24, Aᵣ(N) = 14 and Aᵣ(O) = 16 to calculate the relative formula mass of Mg(NO₃)₂. First list the number of each atom, then state its molar mass with units.

Check your atom count and masses

There is one Mg, two N and six O. Thus Mᵣ = 24 + 2(14) + 6(16) = 148, with no unit; molar mass is 148 g mol⁻¹. Multiplying only oxygen by the outside 2 would miss one nitrogen atom.

Practise relative and molar masses. For a wrong total, return to the atom count before retrying the arithmetic.