1.4.2—Relative atomic mass (Ar) and relative formula mass (Mr)

Relative atomic mass is a weighted mean relative to one twelfth of the mass of a carbon-12 atom. Relative formula mass is the sum of the relative atomic masses represented in the formula.

$M_r = Σ(A_r × subscript)$

Relative atomic mass and relative formula mass are ratios on the carbon-12 scale, so they have no units. Apply every subscript, including waters of crystallization or repeated ions.

For Ca(OH)₂, include both bracketed groups: Mᵣ = Aᵣ(Ca) + 2[Aᵣ(O) + Aᵣ(H)]. Keep Mᵣ dimensionless; attach g mol⁻¹ only when the same numerical total is used as a molar mass.

How it is assessed

Questions ask for the precise carbon-12-relative definition of relative atomic mass or for a formula mass obtained by summing relative atomic masses.

Command terms

define / determine

What earns marks

Refer to the weighted mean mass of an atom relative to the carbon-12 reference, and include every formula subscript in the mass sum without attaching units to the relative value.

Watch for

Using the mass of an element instead of an atom in the definition, or omitting subscripts when summing a hydrated formula.

Representative question

[Maximum number: 1]

Define the term relative atomic mass $\left(A_{\mathrm{r}}\right)$ .

Retrieve the quantitative chain: count entities with mN_A, sum relative masses from formulae, convert mass with n=m/M, derive formula ratios, use n=VC for solutions, and apply gas-volume ratios at the same temperature and pressure.

Before finalising, check the requested entity, formula subscripts, units, dm³ conversion, balanced-equation coefficients, and any limiting reactant.

1.4.2—Relative atomic mass (Ar) and relative formula mass (Mr)

Relative Atomic and Formula Mass

Applying Relative Masses

Assessment in practice

The Mole Summary

Question 1