paper 3 - practical skills

Question 1

A student has an unknown weak acid of unknown concentration and a bottle of 0.5 mol dm-3 NaOH.
Explain, using brief practical details, how you would determine the Ka of the weak acid. [6]

Answer 1

• Measure 25 cm3 of acid into a beaker/conical flask using volumetric pipette.
• Use a pH probe to measure the initial pH
• Add 1cm3 NaOH from a burette swirl and record the pH
• Repeat until the NaOH is in excess
• Plot a graph of pH 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 the pH at the half neutralisation point.
• At half neutralisation ka = [H+]/pKa=pH
• Ka = 10-pH