Topic 3 of 6
The mole is a counting unit
Connect microscopic particles to measurable mass.
O-Level 6092 (2026) / SEC G3 K324 (2027)
The mole is a counting unit
Connect microscopic particles to measurable mass.
Relative atomic mass, Ar, compares the average mass of an atom with one twelfth of the mass of a carbon-12 atom. Relative molecular mass, Mr, makes the same comparison for a molecule. Add the Ar values in its formula. For an ionic compound, call this relative formula mass instead. These relative masses have no units.
One mole contains the Avogadro constant number of specified entities: approximately 6.02 x 1023. Specify atoms, molecules, ions or formula units. The molar mass M has units g mol-1 and is numerically equal to the relative molecular or formula mass. Use n = m/M and particle count = n x NA.
Worked example
Account for brackets in a formula
Find the relative formula mass of Ca(OH)2 and the amount in 3.70 g. Use Ca = 40, O = 16, H = 1.
- Mr = 40 + 2(16 + 1) = 74.
- Molar mass = 74 g mol-1.
- n = 3.70/74 = 0.0500 mol.
0.0500 mol of Ca(OH)2 formula units; if fully dissolved, this produces 0.100 mol of OH- ions.