RP9 - 'Acids and bases titration'

• Suggest a suitable piece of apparatus to measure out the volume of NaOH solution.

  Explain why it is more suitable than a pipette for this purpose [2 marks]

  • Burette
  • Because it can deliver variable volumes
• Explain briefly why a pH meter should be calibrated before use [1 mark]

  • Idea that over time / after storage meter does not give accurate readings
• DESCRIBE HOW YOU WOULD OBTAIN a PH CURVE FOR THE TITRATION [5 marks]

  • (Ignore references to the use of pipette, filling of burette and calibration of the pH meter)
  • Measure pH of the acid
  • Add alkali in small portions - allow 1cm3 to 2cm3
  • Stir mixture
  • Measure pH after each addition
  • Repeat until alkali is in excess - allow 27-50cm3
  • Add in smaller increments near endpoint (allow 0.1cm3 to 0.5cm3)

  Exam tip:

  • To give full marks, the sequence must follow a logical order
  • (Ignore references to the use of pipette, filling of burette and calibration of the pH meter)
• Suggest how the experimental procedure could be slightly modified in order to give a more reliable value for the end-point [1 mark]

  • Take more pH readings around the endpoint
  • OR add smaller volumes of NaOH near the end-point
• Suggest one reason of anomalous point on the pH graph

  • INEFFECTIVE SWIRLING / STIRRING OF THE MIXTURE
• Two solutions, one with pH 4 and other pH 9 were left open in air.

  pH 9 changed more than the other solution

  Suggest what substance might be present in air to cause pH to change.

  Explain why and how pH 9 solution changes. [3 marks]

  • CO2
  • pH falls / decreases
  • acidic gas OR write an equation CO2 + 2OH- -> CO32- + H2O
• (c) Explain why the pH of an acidic buffer solution remains almost constant despite the addition of a small amount of sodium hydroxide. [2 marks]

  • M1 extra/added OH– removed by reaction with H+ or the acid 1
  • M2 correct discussion of equilibrium shift i.e. HX H+ + X– moves to right
• State why all three of the indicators in Table 6 are suitable for this titration. [1 mark]

  • All have a colour change/pH range within the STEEP/vertical part of the titration curve
• Suggest why the pH probe is washed with distilled water between each of the calibration measurements. [1 mark]

  • Different solutions must not contaminate each other
  • that pH of previous solutions does not contaminate new solution
  • or To wash off any residual solution/substance (which could interfere with the reading)
• Explain why the volume of sodium hydroxide solution added between each pH measurement is smaller as the end point of the titration is approached. [1 mark]

  • To avoid missing the end point
  • Or (Very little pH change per cm3 added at start) ; large change in pH (near end point)