Lab 9 Determination of the Dissociation Constant of a Weak Acid (2).docx

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Lab 9
Determination of the Dissociation Constant of a Weak Acid
USF-SM, General Chemistry II Laboratory
To become familiar with the operation of a pH meter and use pH to determine the magnitude of
the equilibrium constant of a weak acid.
Apparatus
pH meter with electrodes
250 mL beakers (2)
100 mL beakers (2)
Burette
25 mL pipet and pipet bulb
Burette clamp and ring stand
Ring stand and ring
RO water bottle
Chemicals
Standard buffer solution (pH = 4.00 and pH = 7.00 Buffer)
Unknown solution of a weak acid
0.1 M NaOH
Introduction
Acid - Base Equilibria
According to the Br  nsted- Lowry acid- base theory, the strength of an acid is related to its ability
to donate protons. All acid- base reactions are then competitions between bases of various
strengths for these protons. For example, the strong acid HCl reacts with water according to
Equation [1].
HCl(aq) + H
2
O(l)  H
3
O+(aq) + Cl-(aq) {1}
This acid is a strong acid and is completely dissociated, in other words, 100% dissociated in dilute
aqueous solution. Consequently, the [H
3
O+] concentration of 0.1 M HCl is 0.1 M .
By contrast, acetic acid, CH
3
COOH (abbreviated HOAc), is a weak acid and is only slightly
dissociated to form hydronium and acetate (abbreviated OAc
-
) ions, as shown in equation {2}.
H
2
O(l) + HOAc(aq) ⇌ H
3
O+(aq) + OAc-(aq) {2}
Therefore, its acid dissociation constant, as shown by Equation {3}, is small.
K =
[ ?
3
?
+
][ ? ? ?
−
]
= 1 . 8 × 10
− 5
{3}
a
[ ? ? ?? ]
Acetic acid only partially dissociates in aqueous solution, an appreciable quantity of undissociated
acetic acid remains in solution.

For the general weak acid HA in aqueous solution, the dissociation reaction and dissociation
constant expression are,
HA(aq) + H
2
O(l) ⇌ H
3
O+(aq) + A-(aq) {4}
K =
[
?
3
?
+
]
[?
−
]
{5}
a
] [??
Recall that pH is defined as pH = -log [H
3
O+] {6}
Solving Equation {5} for [H
3
O+] and substituting this quantity into equation {6} yield
[H
3
O+]= K
] [??
{7}
] [?
-log [H
3
O+] = -logK
a
-log
] [??
[? ]
{8}
pH = pK
a
-log
] [??