Practical 9 pH changes when a weak acid & strong base react

• pH

  1. Start by calibrating a pH meter by placing the pH meter in a buffer solutions with known pH values e.g. 4.00, 7.00, 10.00. Then plot the calibration curve
  2. Measure 25 cm3 of ethanoic acid using a pipette and transfer into a 250 cm3 beaker
  3. Fill a burrette with sodium hydroxide solution
  4. Place the pH meter in the beaker containing ethanoic acid
• pH pt2

  5. Add the NaOH in 2 cm3 portion to the ethanoic acid whilist swirling and record the pH

  6. As you approach the end point add the NaOH in smaller portions of 0.2 cm3

  7. Correct all the pH values using the calibration curve

  8. Plot the pH curve of pH on the y axis and volume of NaOH on the x axis
• State why a pH meter must be calibrated before it is used

  As it looses accuracy over time
• Explain how a pH meter can be calibrated

  The pH meter is placed into solutions of known pH’s e.g. 4.00, 7.00, 10.00 and then a graph is drawn between the pH of the solution and the pH meters values for the solutions
• Explain how you would use a pH curve to calculate the acid dissociation constant ka

  First find the volume used by going to where the curve is steepest (at the endpoint). Because pH = pka at half equivalence you half the volume found and find the pH at that point.

  Then pka = the pH found and then subsitute pka into

  pka = -log10 ka / 10^ -pka = ka

  and so ka is found