DETERMINATION OF THE DISSOCIATION CONSTANT OF A WEAK ACID FROM pH MEASUREMENTS

DETERMINATION OF THE DISSOCIATION CONSTANT OF A WEAK ACID FROM pH
MEASUREMENTS
Aim
The aim of this experiment is to determine the pK
a
for benzoic acid using pH measurements.
Pre-lab questions
1. A weak acid is one which doesn't ionise fully when it is dissolved in water. A weak acid is more
reluctant to donate protons. And they have a high pK
a
such that they release all their hydrogen atoms
when dissolved in water.
2. i) K
a
is a measure of acid strength.
K
a
is defined for the reaction:
HA( aq ) + H
2
O( l ) H
3
O + ( aq ) + A – ( aq )
With an equilibrium constant:
A larger equilibrium constant means that the reaction has a greater tendency to drive towards the
products side (more H
3
O
+
and A
–
, less HA). It can be seen that the greater is the extent of dissociation,
the greater will be the value of K
a
. Or, the stronger is the acid, the greater will be its K
a
. Another way
of saying this is that the acid has a stronger tendency to donate its H + , which is the definition of a
stronger acid.
ii) pK
a
is the negative log of the acid dissociation constant (Ka).
pK
a
=−logK
a
Since, pKa is negative log of Ka, it's values will be greater for weaker acid. The stronger the acid, the
weaker will be its conjugate base. (The larger the pK
a
of the conjugate acid, the stronger the base).
3. pK
a
= −logK
a
pK
a
= −log(5.6 x 10 -4 )
pK
a
= 3.25
4. pH = pK
a
+ log([A - ]/[HA])
However, at one half of the equivalence point, half of the weak acid (HA) has been converted into its
conjugate base (A - ), so:
[HA] = [A - ] and log([A - ]/[HA]) = 0
Therefore ,
pH = pK
a
(at half the equivalence point)
5. pH = pK
a

4.8 = -log
10
K
a
-4.8 = log
10
K
a
K
a
= 10 -4.8 = 1.58 x 10 -5
6. n (KHP) = cV c (KHP) = 0.1 M, V (KHP) = 100 x 10
-3
L
n (KHP) = (0.1)( 100 x 10
-3
) = 0.01 mol
m (KHP) = n x Mr n (KHP) = 0.01 mol, Mr (KHP) = 204.17 g/mol
m (KHP) = (0.01)(204.17) = 2.0417 g
Procedure
About 150 mL of distilled water was boiled in a 250 mL beaker and cooled to room temperature. The
beaker was covered with a petri dish during cooling. 0.1256 g of benzoic acid was accurately weighed
into a clean, dry 250 mL beaker. 40 mL of the boiled, cooled distilled water was added using a
measuring cylinder and stirred to dissolve. (It was necessary to warm the mixture in order to dissolve
the acid. However, the solution was cooled to room temperature before it was transferred into the