The Acid Dissociation Constant, Ka

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THE ACID DISSOCIATION
CONSTANT, KA
THE ACID DISSOCIATION CONSTANT
When a weak acid dissolves in water, it does not
completely dissociate.
 The concentration of the hydronium ions,
and the concentration of the conjugate base
of the acid that is formed in solution,
depend on the initial concentration of the
acid and the amount of acid that
dissociates.

THE ACID DISSOCIATION CONSTANT
You can represent any weak monoprotic acid
with the general formula HA.
 Thus you can express the equilibrium of a weak
monoprotic acid in aqueous solution as follows:
HA(aq) + H2O(l)
H3O+(aq) + A-(aq)
 The equilibrium constant expression for this
reaction is:
Kc = [H3O+][A-]
[HA][H2O]

THE ACID DISSOCIATION CONSTANT
In dilute solutions, the concentration of water is
almost constant.
 Multiplying both sides of the equilibrium
expression by [H2O] gives the product of two
constants on the left side.
 This new constant is called the acid
dissociation constant, Ka.
Kc[H2O] = Ka = [H3O+][A-]
[HA]

KA OF A WEAK ACIDS

Some Acid Dissociation Constants for Weak Acids
at 25oC
Acid
Formula
Ka
Acetic acid
CH3COOH
1.8 x 10-5
Chlorous acid
HClO2
1.1 x 10-2
Formic acid
HCOOH
1.8 x 10-4
Hydrocyanic acid
HCN
6.2 x 10-10
Hydrofluoric acid
HF
6.6 x 10-4
Water
H2 O
1.0 x 10-14
Lactic acid
CH3CHOHCOOH
1.4 x 10-4
Nitrous acid
HNO2
7.2 x 10-4
Phenol
C6H5OH
1.3 x 10-10
SOLVING EQUILIBRIUM PROBLEMS THAT
INVOLVE ACIDS AND BASES
Problems that involve the concentrations of ions
formed in aqueous solutions are considered to be
equilibrium problems.
 The steps for solving acid and base equilibrium
problems are similar to the steps you learned in
the previous unit for solving equilibrium
problems.

SOLVING EQUILIBRIUM PROBLEMS THAT
INVOLVE ACIDS AND BASES
1.
2.
3.
Write the chemical equation.
Use the chemical equation to set up an ICE
table for the reacting substances
whenever the initial acid concentrations
are involved.
Enter any values that are given in the
problem.
SOLVING EQUILIBRIUM PROBLEMS THAT
INVOLVE ACIDS AND BASES
4.
5.
6.
For problems that give the initial
concentration of the acid, [HA], compare
the initial concentration of the acid with
the acid dissociation constant, Ka.
If [HA]/Ka > 500, the change in the initial
concentration is negligible and can be
ignored.
If [HA]/Ka < 500, the change in the initial
concentration may not be negligible.
Solving the equilibrium constant
expression for x will be more complex
(possibly have to use quadratic equation).
PERCENT DISSOCIATION
The percent dissociation of a weak acid is
the fraction of acid molecules that
dissociate compared with the initial
concentration of the acid, expressed as a
percent.
 The percent dissociation depends on the value of
Ka for the acid, as well as the initial
concentration of the weak acid.

SAMPLE PROBLEM
Propanoic acid, CH3CH2COOH, is a weak
monoprotic acid that is used to inhibit mould
formation in bread. A student prepared a 0.10
mol/L solution of propanoic acid and found that
the pH was 2.96. What is the acid dissociation
constant for propanoic acid? What percent of its
molecules were dissociated in the solution?
 What is required?



Ka and percent dissociation for propanoic acid
What is given?


Initial [CH3CH2COOH] = 0.10 mol/L
pH = 2.96
SAMPLE PROBLEM

Step 1
Use the equation for the dissociation of propanoic
acid in water to set up an ICE table.
CH3CH2COOH(aq) + H2O(l)
Initial
Change
Equil
CH3CH2COO-(aq) + H3O+
0.10 mol/L
0
0
-x
+x
+x
0.10 - x
x
x
SAMPLE PROBLEM
Step 2
Ka = [CH3CH2COO-][H3O+]
[CH3CH2COOH]
= (x)(x)
(0.10 - x)

Step 3
The value of x is equal to [H3O+] and [CH3CH2COO].
[H3O+] = 10-2.96
= 1.1 x 10-3 mol/L

SAMPLE PROBLEM
Step 4
Ka =
(1.1 x 10-3)2
0.10 - (1.1 x 10-3)
= 1.2 x 10-5

Step 5
Percent dissociation = 1.1 x 10-3 mol/L x 100
0.10 mol/L
= 1.1 %

PRACTICE PROBLEMS

Questions 5-10, pages 591-592

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