PAG 2 - Acid Base Titration

• What apparatus is used in a titration to accurately measure volumes?

  • Burette used to measure the volume of solution added
  • Pipette and pipette filler to measure a specific volume of solution into the conical flask
• Describe the 'weighing-by-difference' method.

  • Add roughly the desired mass of a substance to a weighing bottle and weigh
  • Transfer the substance to a beaker / conical flask
  • Reweigh the weighing bottle
  • The difference between the masses is the mass of the solid
• Describe how to carry out an acid-base titration.

  1. Use pipette to add 25cm³ of acid to conical flask. Add few drops of indicator
  2. Pour alkali into burette. Record initial burette volume.
  3. Complete trial titre. Conical flask should be swirled constantly above a white tile. Stop adding alkali as soon as end point is reached. Record final burette volume.
  4. Repeat titration until concordant results obtained. Add alkali drop by drop near end point.
• When transferring a solution from one beaker to another, how can you ensure as much of the dissolved substance has been transferred as possible?

  Use distilled water to wash any leftover solution from the old apparatus into the new beaker
• How do you accurately fill a volumetric flask to the graduated mark?

  Fill so the bottom of the meniscus rests on the graduated mark.

  The meniscus is the curve of the liquid.
• What is a standard solution?

  A solution of known concentration
• Describe how to make a standard solution of NaHCO₃.

  1. Weigh out specified amount of NaHCO₃
  2. Add small volume of distilled water to beaker containing NaHCO₃ to dissolve it
  3. Use funnel to add solution to 250cm³ volumetric flask
  4. Rinse beaker with distilled water and add washings to flask
  5. Make flask up to graduation mark with distilled water
  6. Add stopper and shake flask
• Suggest 2 indicators that could be used in an acid-base titration.

  1. Phenolphthalein
  2. Methyl orange
• State the colour of phenolphthalein in

  1. acidic conditions - pink
  2. basic conditions - colourless
• State the colour of methyl orange in

  1. acidic conditions - red
  2. basic conditions - yellow

  (orange at the end point of a titration)
• What are concordant results in a titration?

  Titres that are within 0.1 cm3 of each other
• What degree of precision should burette readings be recorded to?

  To the nearest 0.05 cm3
• Which results are used when calculating a mean titre?

  2 concordant results (within 0.10 cm3 of each other)
• When completing a titration, what piece of apparatus is used to make the colour change is easier to observe?

  • A white tile
  • This is placed underneath the conical flask
• What 3 things are required to take an accurate burette reading?

  1. Read from the bottom of the meniscus
  2. Allow any liquid on the walls of the burette to settle before taking a reading
  3. Ensure there are no air bubbles
• What is the end point of a titration?

  The first point at which the indicator changes colour (permanently)
• Why is the conical flask swirled during a titration?

  To ensure all the reactants are combined so that the reaction is complete
• Why is it better to have a titre volume of 25 cm3 than 10 cm3?

  The larger the titre volume, the smaller the percentage error
• What equation links number of moles and concentration?

  Number of moles = concentration x volume

  n = C x V
• If you know the volume of acid required to neutralise an alkali, how could you calculate the concentration of the acid, given the alkali concentration and volume?

  • Calculate the number of moles of the alkali using the known volume and concentration (n = C x V)
  • Use the chemical equation to work out the ratio of acid and alkali that react and hence work out how many mole of acid have reacted
  • Divide the moles of acid by the volume used in neutralisation