The mole is the unit for amount of substance. The number of particles in a substance can be found using the Avogadro constant. The mass of product depends upon the mass of limiting reactant.

The concentration of a solution can be shown in g/dm^{3} or mol/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:

0.5 mol of sodium hydroxide is dissolved in 2 dm^{3} of water. Calculate the concentration of the sodium hydroxide solution formed.

Concentration =

Concentration = 0.25 mol/dm^{3}

Volumes used in concentration calculations must be in dm^{3}, not in cm^{3}. It is useful to know that 1 dm^{3} = 1000 cm^{3}. This means:

- divide by 1000 to convert from cm
^{3}to dm^{3} - multiply by 1000 to convert from dm
^{3}to cm^{3}

For example, 250 cm^{3} is 0.25 dm^{3} (250 ÷ 1000). It is often easiest to convert from cm^{3} to dm^{3} before continuing with a concentration calculation.

- 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 ÷ 1000 = 0.1 dm

^{3}Concentration of acid = 0.02 mol ÷ 0.1 dm

^{3}= 0.2 mol/dm

^{3}

The relative formula mass of the solute is used to convert between mol/dm^{3} and g/dm^{3}:

- to convert from mol/dm
^{3}to g/dm^{3}, multiply by the relative formula mass - to convert from g/dm
^{3}to mol/dm^{3}, divide by the relative formula mass

Remember: the molar mass is the *A*_{r} or *M*_{r} in grams per mol.

Calculate the concentration of 0.1 mol/dm^{3} sodium hydroxide solution in g/dm^{3}. (*M*_{r} of NaOH = 40)

Concentration = 0.1 × 40

= 4 g/dm^{3}

- Question
Calculate the concentration of 7.3 g/dm

^{3}hydrochloric acid in mol/dm^{3}. (*M*_{r}of HCl = 36.5)Concentration = 7.3 ÷ 36.5

= 0.2 mol/dm

^{3}