218 ❯ STEP 4. Review the Knowledge You Need to Score High
ACID STRENGTH BASE STRENGTHACID
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 −StrongStrongFigure 15.1 Conjugate acid–base pair strengths.The [H 2 O] is assumed to be a constant and is incorporated into the Ka value. It is not
shown in the equilibrium constant expression.
Since this is the equilibrium constant associated with a weak acid dissociation, this
particular Kc is most commonly called the acid dissociation constant, Ka. The Ka expres-
sion is then:
=+−
K[HO][A]
a [HA]
3Many times the weak acid dissociation reaction will be shown in a shortened notation,
omitting the water: +−+=+−
HA(aq)H(aq) A(aq)withK[H ][A]
a [HA]The greater the amount of dissociation is, the larger the value of Ka. Table 15.1, on the
next page, shows the Ka values of some common weak acids.
Here are a couple of tips: For every H+ formed, an A- is formed, so the numerator of
the Ka expression 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
Minitial is the initial concentration of the weak acid. This is true because for every H+ thatTIP