methods

• Titration


  The volumes of acids and alkali solutions that react with each other can be measured by titration using a suitable indicator
• Required practical: Determination of the reacting volumes of solutions of a strong acid and a strong alkali by titration

  • Involves using appropriate apparatus to make and record a range of measurements accurately, including volume of liquids
  • Involves determining the concentration of one of the solutions in mol/dm3 and g/dm3 (Higher tier only)
  • It is important to make and record accurate measurements and take account of health and safety considerations
• Carrying out the practical

  1. Use the pipette and pipette filler to add 25 cm3 of alkali to a clean conical flask

  2. Add a few drops of indicator and put the conical flask on a white tile

  3. Fill the burette with acid and note the starting volume

  4. Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix

  5. Stop adding the acid when the end-point is reached (when the indicator first permanently changes colour)

  6. Note the final volume reading

  7. Repeat steps 1 to 5 until you get concordant titres
• End-point


  In a titration, the point at which the indicator first permanently changes colour
• Concordant titres


  Titres within 0.10 cm3 of each other
• Titre


  Volume of one reactant needed to react completely with the other reactant in a titration
• Analysis


  1. At least two concordant titres should be ticked (✔)

  2. Calculate the mean titre
• The mean titre is the volume of acid that exactly reacts with the sodium hydroxide solution of unknown concentration
• Readings should be recorded to two decimal places, ending in 0 or 5 (where the liquid level is between two graduations on the burette)
• More accurate results are obtained if acid is added drop by drop near to the end-point
• Burette


  A piece of apparatus used to add varying but measured volumes of solution during a titration
• Pipette


  A piece of apparatus used to measure accurate and repeatable volumes of liquid. Also called a volumetric pipette
• The pipette allows the same volume of alkali to be added each time, helping to make the results repeatable
• Eye protection must be worn
• Dilute sodium hydroxide solution causes skin and serious eye irritation
• Spilling hydrochloric acid while filling the burette causes eye irritation
• Steps to obtain accurate results

  1. Make sure the burette is vertical

  2. Take the readings from the bottom of the meniscus

  3. Near to the end-point, rinse the inside of the flask with distilled water

  4. Add the acid drop by drop
• moles= volume x concentration