Practical 9- Paper 1 And 3: The Change In PH

Question 1

Describe a practical that allows you to determine the Ka of a weak acid practically

Answer 1

• measure 25cm3 of acid into a beaker using a volumetric pipette
• use a pH probe to measure the initial pH
• add 1cm3 of a 0.1mol dm-3 NaOH from a burette, swirl and record the pH
• repeat until NaOH is in excess
  -plot a graph against vol NaOH added in cm3
• use the vertical section to find the volume of NaOH needed for neutralisation
• the half neutralisation point is half the volume of neutralisation
• read off pH at half neutralisation point
• at half neutralisation Ka= [H+]/pKa
• Ka=10-pH