5 Steps to a 5 AP Chemistry

The [H 2 O] is assumed to be a constant and is incorporated into the Kavalue. It is not

shown in the equilibrium constant expression.

Since this is the equilibrium constant associated with a weak acid dissociation, this par-

ticular Kcis most commonly called the acid dissociation constant, Ka. The Kaexpression

is then:

Many times the weak acid dissociation reaction will be shown in a shortened notation,

omitting the water:

The greater the amount of dissociation is, the larger the value of Ka. Table 15.1 shows

the Kavalues of some common weak acids.

Here are a couple of tips: For every H+formed, an A−is formed, so the numerator of

the Kaexpression can be expressed as [H+]^2 (or [A−]^2 , although it is rarely done this way).

Also, the [HA] is the equilibrium molar concentration of the undissociated weak acid, not

its initial concentration. The exact expression would then be [HA] =Minitial− [H+], where

Minitialis the initial concentration of the weak acid. This is true because for every H+that is

H A(a q) H a q) + A a q) wit h

HA

HA]

+

a

+

 ((

[][]

[

−

−

K =

Ka^3

HO A+

HA]

=

[][]−

[

Equilibrium  217

ACID STRENGTH BASE STRENGTH

ACID

HCl

H 2 SO 4

HNO 3

H 3 O+

HSO 4 −

H 2 SO 3

H 3 PO 4

HF

CH 3 COOH

H 2 CO 3

H 2 S

HSO 3 −

H 2 PO 4 −

NH 4 +

HCO 3 −

HPO 42 −

H 2 O

OH−

BASE

Cl−

HSO 4 −

NO 3 −

H 2 O

SO 42 −

HSO 3 −

H 2 PO 4 −

F−

CH 3 COO−

HCO 3 −

HS−

SO 32 −

HPO 42 −

NH 3

CO 32 −

PO 43 −

OH−

O^2 −

Strong

Strong

Figure 15.1 Conjugate acid–base pair strengths.

TIP