Moles and masses - Higher

The mass of a substance can be calculated from the number of moles, and the number of moles of a substance can be calculated from its mass. The link between the two quantities is the Mr, relative formula mass (or Ar, relative atomic mass for atoms and metals).

Calculating masses

The mass of a given number of moles of substance is calculated using: \(mass~in~g = \textit{M}_{\textup{r}}\times~number~of~moles~or~mass =\textit{M}_{\textup{r}}~\times~mol\)

One way to remember this is 'mass equals mister mole'.


Calculate the mass of 0.25 mol of carbon dioxide molecules . (Mr of CO2 = 44)

mass = 44 × 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 0.10 mol of iron. (Ar of Fe = 56)

mass = 56 × 0.10

= 5.6 g

Calculating moles

The amount of a given mass substance is calculated using: \(amount~in~mol=\frac{mass~in~g}{\textit{M}_\textup{r}}\)

Again, use Ar instead of Mr for metals or a separate atom.


Calculate the amount of carbon atoms in 6.0 g of carbon. (Ar of C = 12)

amount = \(\frac{\textup{mass}}{{A}_\textup{r}}\)

amount \(=\frac{\textup6.0}{\textup12}\)

= 0.5 mol