The concentration of a solution can be shown in g/dm3 or mol/dm3. It is often more useful to know the concentration of a reactant in mol/dm3 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 dm3 of water. Calculate the concentration of the sodium hydroxide solution formed.
Concentration = 0.25 mol/dm3
Volumes used in concentration calculations must be in dm3, not in cm3. It is useful to know that 1 dm3 = 1000 cm3. This means:
For example, 250 cm3 is 0.25 dm3 (250 ÷ 1000). It is often easiest to convert from cm3 to dm3 before continuing with a concentration calculation.
100 cm3 of dilute hydrochloric acid contains 0.02 mol of dissolved hydrogen chloride. Calculate the concentration of the acid in mol/dm3.
Volume of acid = 100 ÷ 1000 = 0.1 dm3
Concentration of acid = 0.02 mol ÷ 0.1 dm3
= 0.2 mol/dm3
The relative formula mass of the solute is used to convert between mol/dm3 and g/dm3:
Remember: the molar mass is the Ar or Mr in grams per mol.
Calculate the concentration of 0.1 mol/dm3 sodium hydroxide solution in g/dm3. (Mr of NaOH = 40)
Concentration = 0.1 × 40
= 4 g/dm3
Calculate the concentration of 7.3 g/dm3 hydrochloric acid in mol/dm3. (Mr of HCl = 36.5)
Concentration = 7.3 ÷ 36.5
= 0.2 mol/dm3