Determining the acid dissociation constant (Ka) for a weak acid
Aim
The purpose of this experiment is to accurately demonstrate a reading of pH of a half-
neutralised solution of ethanoic acid and from this determine a value for Ka.
Introduction
You will first accurately determine the titre of sodium hydroxide solution required to
neutralise 25.0 cm3 of 1.0M ethanoic acid. You will then neutralise exactly half of a new
sample of 25.0 cm3 of ethanoic acid with sodium hydroxide and accurately measure the
pH of the resulting solution in order to determine Ka.
Chemicals and Equipment
o ethanoic acid (approx. 0.1M)
o sodium hydroxide solution (0.1M)
o safety spectacles
o 1 burette, 50 cm3, and stand
o 1 funnel, small
o pipette, 25.0 cm3, and pipette filler
o 1 beaker, 250 cm3
o 1 conical flask, 250 cm3
o distilled water
o pH meter (accurately calibrated)
CAUTION:
Sodium hydroxide solution is corrosive. You must wear safety spectacles throughout the
experiment.
Method
1. Clamp the 50 cm3 burette to the stand. Using a funnel, rinse and fill the burette
with 0.1M sodium hydroxide solution.
2. Using a pipette filler, rinse the pipette with some of the ethanoic acid and transfer
precisely 25.0 cm3 of the solution into a 250 cm3 conical flask.
3. Add 2-3 drops of phenolphthalein indicator solution to the conical flask and swirl
to mix.
4. Run sodium hydroxide solution from the burette, with swirling until the solution
just turns pink and is permanent.
5. Repeat the titration until you have obtained concordant titres. Calculate the mean
titre (V cm3).
6. Pipette precisely 25.0 cm3 of the ethanoic acid solution this time into a 250 cm3
beaker. Do not add phenolphthalein indicator.
7. Run exactly V/2 cm3 of sodium hydroxide solution from the burette into the
beaker of ethanoic acid. This is the volume required to neutralise exactly half of
the ethanoic acid.
8. Swirl gently to ensure that the solutions are well mixed.
9. Place an accurately calibrated pH meter into the beaker so that the tip of the
probe is completely immersed in the solution you have made.
10. When the reading is stable, read and record the final pH of the solution in the
beaker.
Aim
The purpose of this experiment is to accurately demonstrate a reading of pH of a half-
neutralised solution of ethanoic acid and from this determine a value for Ka.
Introduction
You will first accurately determine the titre of sodium hydroxide solution required to
neutralise 25.0 cm3 of 1.0M ethanoic acid. You will then neutralise exactly half of a new
sample of 25.0 cm3 of ethanoic acid with sodium hydroxide and accurately measure the
pH of the resulting solution in order to determine Ka.
Chemicals and Equipment
o ethanoic acid (approx. 0.1M)
o sodium hydroxide solution (0.1M)
o safety spectacles
o 1 burette, 50 cm3, and stand
o 1 funnel, small
o pipette, 25.0 cm3, and pipette filler
o 1 beaker, 250 cm3
o 1 conical flask, 250 cm3
o distilled water
o pH meter (accurately calibrated)
CAUTION:
Sodium hydroxide solution is corrosive. You must wear safety spectacles throughout the
experiment.
Method
1. Clamp the 50 cm3 burette to the stand. Using a funnel, rinse and fill the burette
with 0.1M sodium hydroxide solution.
2. Using a pipette filler, rinse the pipette with some of the ethanoic acid and transfer
precisely 25.0 cm3 of the solution into a 250 cm3 conical flask.
3. Add 2-3 drops of phenolphthalein indicator solution to the conical flask and swirl
to mix.
4. Run sodium hydroxide solution from the burette, with swirling until the solution
just turns pink and is permanent.
5. Repeat the titration until you have obtained concordant titres. Calculate the mean
titre (V cm3).
6. Pipette precisely 25.0 cm3 of the ethanoic acid solution this time into a 250 cm3
beaker. Do not add phenolphthalein indicator.
7. Run exactly V/2 cm3 of sodium hydroxide solution from the burette into the
beaker of ethanoic acid. This is the volume required to neutralise exactly half of
the ethanoic acid.
8. Swirl gently to ensure that the solutions are well mixed.
9. Place an accurately calibrated pH meter into the beaker so that the tip of the
probe is completely immersed in the solution you have made.
10. When the reading is stable, read and record the final pH of the solution in the
beaker.
