Cliffs AP Chemistry, 3rd Edition

Experiment 11: Determination of Appropriate

Indicators for Various Acid-Base Titrations

and

Experiment 19: Preparation and Properties

of Buffer Solutions

Background: Strong acids generally dissociate into ions nearly completely. The hydronium
ion concentration of a strong monoprotic acid solution is therefore essentially equal to the con-
centration of the acid in solution*. In mono-hydroxy strong bases, the hydroxide ion generally
dissociates nearly completely and the hydroxide ion concentration is equivalent to the concen-
tration of the base in solution**. In 1909, Sören Sörenson proposed the system that we use to-
day in measuring the concentration of acids and bases. This system is based on exponents to
overcome the difficulties of dealing with small numbers.

log

log

(^110)
(^110)
pH log
H
HHO
pOH log
OH
OH OH
3 pH
pOH
==- =
==- =

• ++-

---

7

77

7

77

A

AA

A

AA

Since Kw= 1.0 × 10 –14= (H+) ⋅(OH–); logKw= –14.00 = log(H+) + log(OH–); [–log(H+)] +

[–log(OH–)] = –[–14.00], then pH + pOH = 14.00***

For weak acids, those that do not dissociate completely (usually 5% dissociation or less),
hydronium ion concentration is not equal to the concentration of the anion of the acid in solu-
tion (it is less). In weak bases, as well, the concentration of the hydroxide ion is not equal to
the concentration of the cation of the base (it is less).

In the case of a weak acid, acetic acid, which is represented by HAc, the dissociation reaction
can be written as:

HAc]aqg++H O (^23) ]lg+Ac]]-+aqggH O aq
The equilibrium expression is therefore:
K.^5
HAc
HO Ac
a 18 10 M
3
==#
+-

• 6

77

@

AA

The concentration of the acetic acid that has not dissociated is considered to be approximately
equal to the initial concentration of the acid because the extent of the dissociation of weak
acids is small. However, often it is not negligible and then to solve for the equilibrium concen-
trations, use of the quadratic equation or other special mathematics is needed.

* more accurately as the anion of the acid

** more accurately as the cation of the base

*** temperature dependent

Part III: AP Chemistry Laboratory Experiments