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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 monoprotic acid is dissolved in water, it dissociates only to a very small extent (i.e.
favors the reactants). 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 and all the H3O
+
ions are 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
acid in the sample by the moles of acid present in this sample, which is stoichiometrically equal to
the moles of base required to reach the equivalence point.
