Moles and masses - Higher

There is a relationship between a relative atomic mass (Ar) or relative formula mass (Mr), and the mass of one mole of a substance:

The mass of 1 mol of a substance is its Ar or Mr in grams (g).

Some examples are given in the table below.

SubstanceFormulaAr or MrMass of 1 mol (g)
OxygenO22 × 16.0 = 32.032.0
Calcium carbonateCaCO340.1 + 12.0 + (3 × 16.0) = 100.1100.1

This relationship is useful because it makes it possible to work out the mass of reactants needed to make a particular mass of product. It is also possible to work out which reactant will be used up.

Calculating number of moles

The number of moles of a given mass substance is calculated using:

number of moles = \(\frac{mass~(g)}{relative~formula~mass (g)}\)

Use Ar instead of Mr for metals or separate atoms.


Calculate the number of moles of carbon atoms in 6.0 g of carbon. (Relative atomic mass: C = 12.0)

number of moles = \(\frac{mass}{A_r}\)

number of moles = \(\frac{6.0}{12.0}\)

= 0.50 mol

Calculating masses

To calculate the mass if given the number of moles and the relative formula mass, the equation must be rearranged to make mass the subject.

The mass of a given number of moles of substance is calculated using: mass (g) = relative formula mass (g) × number of moles

One way to remember this is ‘mass equals mister mole’.


Calculate the mass of 0.25 mol of carbon dioxide molecules. (Relative formula mass: CO2 = 44.0)

mass = 44.0 × 0.25

= 11 g

The calculation is the same if a substance is a metal or exists as separate atoms, but its Ar is used instead of an Mr.



Calculate the mass of 0.125 moles of iron. (Relative atomic mass: Fe = 55.8)

mass = 55.8 × 0.125

= 6.98 g (to 3 significant figures)