8.6 Acid-Base Properties of
Salt Solutions
Learning Goals …
… determine the pH of a solution containing a salt
• A salt is an ionic compound containing a cation (+ ion) and
an anion (- ion)
• they are produced in neutralization reactions
• salts can be acidic, basic or neutral
• the pH is dependent upon whether the cation and anion
are conjugates of strong or weak acids and bases
• the cation comes from the base and the anion comes from
the acid
Strong Acids:
Weak Acids:
Strong Bases:
Weak Bases:
HCl HBr HI HNO3 H2SO4 HClO4
all other H ___
metal + OHall other, ex NH3 CH3NH2 C5H5N
Recall:
A strong acid or base will have a conjugate with negligible
acid/base properties (it will not react with water)
HCl
+ H2O
strong acid
NaOH
strong base
 H3O+
+
Clnegligible conj base
Cl- + H2O  NR

Na+
negligible conj acid
Na+ + H2O  NR
+
OH-
A weak acid or base will produce conjugate that has weak
acid/base properties (it will react with water)
HCH2H5O2
weak acid
+ H2O
 H3O+ +
CH2H5O2weak conj base
CH2H5O2- + H2O  HCH2H5O2 + OHpH > 7
NH3
+ H2O
weak base
 NH4+ +
OHweak conj acid
NH4+ + H2O  NH3 + H3O+
pH < 7
Salts that Produce Neutral Solutions…
Ex: NaCl, Ca(NO3)2, KBr
• Cation (eg Na+) is the conjugate of a strong base (eg NaOH)
and therefore has negligible acidic properties.
• Anion (eg Cl-) is the conjugate of a strong acid (eg HCl) and
therefore has negligible basic properties.
• Neither of the ions act as an acid or a base and thus the
pH is neutral
• a salt produced in the reaction of a STRONG ACID and
STRONG BASE will be NEUTRAL
Salts that Produce Acidic Solutions…
Ex: NH4Cl
• Cation (NH4+) is the weak conjugate acid of a weak base
(NH3). This conjugate acid would thus react with water to
produce H3O+.
NH4+ + H2O  NH3 + H3O+
• Anion (Cl-) is the conjugate of a strong acid (eg HCl) and
therefore would have negligible basic properties.
• Cation acts as an acid; anion has no effect on pH
• a salt produced in the reaction of a STRONG ACID and
WEAK BASE will be ACIDIC
Ex)
What is the pH of a 0.35 M NH4Cl solution? (Kb for
ammonia is 1.8x10-5)
NH4Cl(aq)
0.35 M

NH4+(aq)
0.35 M
NH4+ + H2O 
i
c
e
0.35 M
-x
0.35 - x
NH3
0
+x
x
+
+
Cl-(aq)
H3O+
0
+x
x
Ka = Kw = 1x10-14 = 5.56x10-10
Kb
1.8x10-5
Ka value
needed for
NH4+
Ka =
(x)(x)
(0.35 – x)
= 5.56x10-10
Can we simplify? 0.35/Ka
> 100 YES
x2
(0.35)
= 5.56x10-10
x2 = 1.946x10-10
x = 1.39x10-5
[H3O+] = x = 1.39x10-5
pH = -log(1.39x10-5)
pH = 4.86
Salts that Produce Basic Solutions…
Ex: NaCH3COO
• Anion (CH3COO-) is the weak conjugate base of a weak acid
(CH3COOH). This conjugate base would thus react with
water to produce OH-.
CH3COO- + H2O  CH3COOH + OH-
• Cation (Na+) is the conjugate of a strong base (eg NaOH)
and therefore will have negligible acidic properties.
• Anion acts as a base; cation has no effect on pH
• a salt produced in the reaction of a WEAK ACID and
STRONG BASE will be BASIC
Ex)
Calculate the pH of a solution that contains 12.5g of
sodium acetate (NaCH3COO) dissolved in 1.0L of H2O.
(Ka for acetic acid is 1.8x10-5)
NaCH3COO(aq)  Na+(aq) + CH3COO-(aq)
n
m
M
C
V
0.152 mol
12.5 g
82.04 g/mol
0.152 M
1.0L
CH3COO-
i
c
e
0.152 M
-x
0.152 - x
+ H2O 
Kb value
needed
0.152 M
CH3COOH
0
+x
x
+
OH-(aq)
0
+x
x
Kb = Kw = 1x10-14 = 5.56x10-10
Ka
1.8x10-5
Kb =
Can we simplify?
> 100 YES
(x)(x)
= 5.56x10-10
(0.152 - x)
x2
= 5.56x10-10
(0.152)
x2 = 8.45x10-11
x = 9.2x10-6
[OH-] = x = 9.2x10-6
pOH = -log(9.2x10-6)
pOH = 5.04
pH = 14 - pOH
pH = 8.96
What about if the salt contained an ion that is AMPHOTERIC?
Ex) Is the solution of KH2PO4 acidic or basic?
The anion H2PO4- can act as both a weak acid and
a weak base
Ka2 = 6.3x10-8
H2PO4- + H2O  HPO42- + H3O+
H2PO4- + H2O  H3PO4 + OH-
Kb = Kw/Ka1
=1.4x10-12
Since the Ka > Kb, H2PO4- will act as an acid and the solution
will be acidic.
Salt Solutions
Sort the following salts based on whether they form acidic,
neutral, or basic solutions when dissolved in water.
NaCl
C5H5NHCl
NH4NO3
LiF
NH4NO3
C5H5NHCl
NaClO
KCN
NaClO
NaCl
LiF
KCN
Self Check
How prepared am I to start my homework? Can I …
… determine the pH of a solution containing a salt
HOMEWORK
p534 #1, 2
p536 #1,2
p539 #1,4