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H-1
EXPERIMENT H: ANALYSIS OF AN
UNKNOWN ACID
OBJECTIVES
In this experiment, the molar mass
and the dissociation constant for an unknown weak acid will be
determined.
THEORETICAL CONSIDERATIONS
Molar mass of a Weak Acid
When a weak acid is dissolved in water,
it dissociates only to a very small extent (i.e. favors the
reactants). For a weak monoprotic
acid, this dissociation is expressed by the following relation:
HA
(aq)
+ H2O
(l)
H3O
+
(aq)
+ A
-
(aq)
(1)
Even though weak acids are only partially
dissociated in water, they react almost completely when
titrated with hydroxide ions.
The hydroxide ion is the most powerful base that can exist in
aqueous solution.
HA
(aq)
+ OH
-
(aq)
H2O
(l)
+ A
-
(aq)
(2)
This process can be explained by Le
Châtelier’s Principle. As the concentration of H3O
+
decreases in solution, because these
ions are being neutralized by the addition of OH
-
, more of the
weak acid will dissociate to compensate
for this stress on the equilibrium. The dissociation of the
weak acid will continue until all
of the HA has dissociated (i.e. been neutralized).
When the equivalence point is reached
in the titration of a weak monoprotic acid with hydroxide
ions, the moles of OH
-
added is equal to the moles of the
weak acid originally present in the
sample. The molar mass of the
unknown acid can be found by simply dividing the mass of the
unknown acid in the sample by the
moles of the unknown acid present in this sample, which is
stoichiometrically equal to the moles
of base required to reach the equivalence point.
Self Ionization of Water and the
pH Scale: A Review
Even when it is very pure, water
contains low concentration of OH
-
(aq)
and H3O
+
(aq)
ions. This is
due to the amphiprotic nature of
water (can both donate and accept H
+
ions). This process can be
demonstrated in the self-ionization
(or autoionization) of water (below):
H2O
(l)
+ H2O
(l)
H3O
+
(aq)
+ OH
-
(aq)
K = [H3O
+
][OH
-
] = 1.00 x 10
-14
(25
C)= K
w
