Calculations involving the mole can be used to determine unknown concentrations, volumes and masses in reactions. One mole of any gas occupies 24 dm3 at room temperature and pressure.

The concentration of a solution can be calculated using:

The solute is the substance that is dissolved. The volume is the volume of the solution or the solvent.

**Example**

5.0 g of sodium hydroxide is dissolved in 250 cm^{3} of water. Calculate the concentration of the sodium hydroxide solution formed in g/dm^{3}.

250 cm^{3} = 250 Ć· 1,000 = 0.250 dm^{3}

= 20 g/dm^{3}

It is often more useful to know the concentration of a reactant in mol/dm^{3} so that the amount of reactant in a given volume can be calculated.

The concentration of a solution can be calculated using:

The units for concentration are sometimes shown as mol dm^{-3} or M (for āmolarā), but this means the same as mol/dm^{3}.

**Example**

0.50 mol of sodium hydroxide is dissolved in 2.0 dm^{3} of water. Calculate the concentration of the sodium hydroxide solution formed in mol/dm^{-3}.

concentration = 0.25 mol/dm^{3}

- Question
100 cm

^{3}of dilute hydrochloric acid contains 0.02 mol of dissolved hydrogen chloride. Calculate the concentration of the acid in mol/dm^{3}.volume of acid = 100 Ć· 1,000 = 0.1 dm

^{3}concentration of acid = 0.02 Ć· 0.1

= 0.2 mol/dm

^{3}