Standard level
The acid dissociation constant, ka, gives a measure of the degree of dissociation of a weak acid molecules into ions, and hence the strength of the acid.
Background
The weak acid dissociation can be represented by the equation:
HA(aq) ⇋ H+(aq) + A-(aq)
And the acid dissociation constant, ka is given by the equation:
Acid dissocation constant
ka = [H+][A-]/[HA]
During a titration of the weak acid by a strong base, the acid is used up and more conjugate base ions, A- are formed.
When exactly half the mol of base needed to neutralise all of the acid is added, then [A-] = [HA], and hence at this point:
ka = [H+]
The definition of pH:
pH = -log10[H+]
Therefore, measurement of pH at the half-equivalence point allows determination of ka.
Chemicals
- Sodium hydroxide, 0.1 mol dm-3
- Ethanoic acid, 0.1 mol dm-3
- Methanoic acid, 0.1 mol dm-3
- Phenolphthalein indicator
Apparatus
- Conical flask, 250ml
- Burette, 50ml
- Pipette, 25 ml
- Pipette filler
- White tile
- Clamp, stand and boss
- pH meter
- Transfer 25 cm3 of the ethanoic acid into a conical flask, add 5 drops of phenolphthalein.
- Titrate against sodium hydroxide from the burette, stirring gently with the pH meter.
- Record the pH after each addition.
- Plot the values on a graph of volume added (x-axis) against pH (y-axis)
- Determine the equivalence volume
- Use the graph to determine the pH value at the half-equivalence point.