Titrations (WJEC GCSE Chemistry: Combined Science)

Using a titration to prepare a soluble salt

• Titrations can be used to prepare a soluble salt e.g. sodium chloride from an acid and alkali
• The acid and alkali are reacted together in a neutralisation reaction
• When the acid and alkali are completely neutralised only a salt and water will be present in the solution

hydrochloric acid + sodium hydroxide → sodium chloride + water

HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)

Steps to prepare a soluble salt

Diagram showing the apparatus needed to prepare a salt by titration

 Method:

• Use a pipette to measure a fixed volume of alkali into a conical flask and add a few drops of an indicator 
• Add the acid into the burette and note the starting volume
• Add the acid very slowly from the burette to the conical flask until the indicator changes to the appropriate colour
• Note and record the final volume of acid in the burette and calculate the volume of acid added (final volume of acid - initial volume of acid)
• Add this same volume of acid into the same volume of alkali without the indicator
• Heat the resulting solution in an evaporating basin to partially evaporate, leaving a saturated solution (crystals just forming on the sides of the basin or on a glass rod dipped in and then removed)
• Leave to crystallise, decant excess solution and allow crystals to dry

Determining the relative concentration of acids and alkalis

• If an alkali of known concentration is used in a titration then the relative concentration of the acid can be determined and vice versa
• This is done by looking at the volumes of each reactant
• If the volume of acid used is the greatest then the acid is less concentrated
  • This is because more acid is required to neutralise the alkali
• If the volume of alkali used is the greatest then the alkali is less concentrated for the same reasons
• This is true of neutralisation reactions where the ratio of acid to alkali is 1:1
• For example, if 30.00 cm³ of 0.2 mol dm^–3 acid is neutralised by 15.00 cm³ of an unknown alkali, the concentration of the alkali must be twice the concentration of the acid
  • This is because only half the volume of the alkali is required to neutralise 30.00 cm³ of acid 

• The method to carry out a titration can be found here

Higher Tier

• Higher tier students need to be able to carry out titration or neutralisation calculations
• This is used to determine the actual concentration of the acid or alkali used in the titration 
• You should be able to work out both the concentration and volume