8.5 Acid-Base Properties of Salt Solutions
Salts in Solution
 A pure substance that dissociates into ions when it
dissolves in water is called an electrolyte.
 Salts can be acidic, basic, or neutral.
 An acidic salt is one that, when it dissolves in water,
increases the concentration of hydrogen ions and thus
form an acidic solution.
 A basic salt increases the concentration of hydroxide
ions and this forms a basic solution in water.
 A neutral salt produces neither hydrogen ions nor
hydroxide ions and thus forms a neutral solution in water.
Acid-Base Properties of Salt Solutions – Hydrolysis
 The reaction of an ion with water to produce an acidic or
basic solution by the production of hydronium or
hydroxide ions.
Recall:
1.
2.
3.
4.
5.
6.
The conjugate base of a strong acid is a very weak base.
The conjugate base of a weak acid is a weak base.
The conjugate base of a very weak acid is a strong base.
The conjugate acid of a very weak base is a strong acid.
The conjugate acid of a weak base is a weak acid.
The conjugate acid of a strong base is a very weak acid.
Salts that Produce Neutral Solutions
KCl(aq)
Since K+(aq) is the cation of a
strong base - KOH(aq) - K+(aq)
does not react with water to form
KOH(aq) and H3O+(aq).
Since Cl-(aq) is the anion
(conjugate base) of a strong acid
- HCl(aq) – Cl-(aq) does not
react with water to form HCl(aq)
and OH-(aq).
No H+(aq) ions are produced; no
OH-(aq) ions are produced.
Neutral solution
Example:
HCl + NaOH  NaCl + H2O
Salts that Produce Basic Solutions
K2C2H3O2(aq)
Since K+(aq) is the cation of a
strong base - KOH(aq) - K+(aq)
does not react with water to form
KOH(aq) and H3O+(aq).
Since C2H3O2-(aq) is the anion
(conjugate base) of a weak acid
– HC2H3O2(aq) – C2H3O2-(aq) is
a sufficiently strong base to react
with water to produce
HC2H3O2(aq) and OH-(aq).
No H3O+(aq) ions are produced;
some OH-(aq) ions are produced.
Basic solution
Example One:
Sodium Acetate
NaC2H3O2
Example Two: Calculate the pH of a 0.30 mol/L sodium nitrite
solution, NaNO2(aq).
Salts that Produce Acidic Solutions
NH4Cl(aq)
Since NH4+(aq) is the cation of a
weak base – NH3(aq) - NH4+ (aq)
is a sufficiently strong acid to
react with water to form NH3(aq)
and H3O+(aq).
Since Cl-(aq) is the anion
(conjugate base) of a strong acid
- HCl(aq) – Cl-(aq) does not
react with water to form HCl(aq)
and OH-(aq).
Some H3O+(aq) ions are produced;
no OH-(aq) ions are produced.
Acidic solution
Example One:
Ammonium Chloride
NH4Cl
Example Two: Calculate the pH of a 0.10 mol/L NH4Cl(aq)
solution.
Salts that Produce Acidic Solutions – Small Ions with
Large Charges
 Highly charged small ions produce acidic solutions– ions
with a small radius but a large positive charge which
results in a high charge density (for example, transition
metals).
 The metal cations become hydrated and act as Bronsted
–Lowry acids and give up protons to water to a
significant extent.
 When a metal ion becomes hydrated, it is bonded to
water molecules.
Example: Al3+ binds to six water molecules
to form Al(H2O)63+ ions. This ion acts as
an acid in a reaction with water.
Al(H2O)63+ + H2O  H3O+ + Al(H2O)5(OH)2+
 Protons from the Al(H2O)63+ ion are transferred easily to
water because one of the partially charged protons on
one of the six H2O molecules bonded to Al3+ appears to
be repelled by the high charge of the metal cation.
 This transfer of protons produces a measurable
concentration of H3O+ which can be used to calculate a
Ka value
Example:
Salts that Act as Acid and Bases
 Some salts contain the cation of a weak base and the
anion of a weak acid – both can hydrolyze
 To find the pH, compare the Ka and Kb of the two ions.
If Ka > Kb
If Ka < Kb
If Ka = Kb
solution is acidic
solution is basic
solution is neutral
Example: Predict whether ammonium phosphate will
hydrolyze and determine if the solution is acidic, basic, or
neutral?
Solution:
 Look at the cation and anion separately to see if either
are weak and will hydrolyze.




(NH4)3PO4(aq)  3NH4+(aq) + PO43-(aq)
NH4+ is the conjugate acid of the weak base, NH3, and
will hydrolyze.
PO43- is the conjugate base of the weak acid, HPO4-, and
will also hydrolyze.
Comparison of Ka (NH4+) and Kb (PO43-) is necessary to
determine the properties of the solution.
To determine the value of Kb, use the relationship:
Ka x Kb = Kw
Kb = Kw
Ka (Ka3 for H3PO4)
Kb =
IN GENERAL:
Type of Salt
Examples
Cation of a
KCl(aq)
Group 1 or
NaCl(aq)
Group 2
NaNO3(aq)
element, other
than Be2+;
anion is from a
strong acid
Cation of a
NaC2H3O2(aq)
Group 1 or
KCN(aq)
Group 2
NaF(aq)
element, other
than Be2+(aq);
anion is from a
weak acid
Cation is a
NH4Cl(aq)
conjugate acid
NH4NO3(aq)
of a weak base;
anion is from a
strong acid
Cation is
NH4C2H3O2(aq)
conjugate acid
NH4CN(aq)
of weak base;
anion is
conjugate base
of weak acid
Cation is highly
Al(NO3)3(aq)
charged metal
FeCl3(aq)
ion; anion is
from strong
acid
Comment
Neither of the
ions acts as an
acid or a base
pH of Solution
Neutral
Anion acts as a
base; cation
has no effect
on pH
Basic
Cation acts as
an acid; anion
has no effect
on pH
Acidic
Cation acts as
an acid; anion
acts as a base
Acidic if Ka > Kb
Basic if Ka < Kb
Neutral is Ka = Kb
Hydrated
cation acts as
an acid; anion
has no effect
on pH
Acidic