Acid-Base Properties of Salts
Type of Salt
1 Cations associated with strong bases
Anions associated with strong acids
pH of Solution example
Neutral
NaCl
KNO3
K2SO4
2 Cations associated with weak bases
Anions associated with strong acids
Acid
NH4Cl
NH4NO3
Base
NaC2H3O2
KNO2
NaOH → Na+ + OHNa+ is the very weak conjugate acid of a strong base
HC2H3O2 → H+ + C2H3O2C2H3O2- is the weak conjugate base of a weak acid
Acid or Base
NH4C2H3O2
NH4F
NH4C2H3O2 → NH4+ + C2H3O2NH3 + H2O → NH4+ + OHNH4+ is the weak conjugate acid of a weak base
HC2H3O2 → H+ + C2H3O2C2H3O2- is the weak conjugate base of a weak acid
The conjugate acid of a weak base is
itself a weak acid.
3 Cations associated with strong bases
Anions associated with weak acids
The conjugate base of a weak acid is
itself a weak base.
4 Anions associated with weak acids
and cations associated with weak
bases are acidic.
The acid–base nature of the salt can
be determined by comparing the
relative values of Ka and Kb for the
ions. In this situation, normally the
cation is acidic and the anion is basic.
explanation
NaOH → Na+ + OHNa+ is the very weak conjugate acid of a strong base
HCl → H+ + ClCl- is the very weak conjugate base of a strong acid
NH3 + H2O → NH4+ + OHNH4+ is the weak conjugate acid of a weak base
HCl → H+ + ClCl- is the very weak conjugate base of a strong acid
Compare the Ka and Kb values for the ions.
acidic if Ka > Kb
basic if Ka < Kb
neutral if Ka = Kb
In this case, Ka for NH4+ is 5.8 x 10-10 and the Kb for
C2H3O2- is 5.6 x 10-10, so Ka > Kb the solution is acidic
5 Cations are small, highly charged (+2 Acid
or +3) metal cations
Anions associated with strong acids
highly charged (+2 or +3) metal
cations are hydrated in water and act
as acidic cations.
6 Metal Oxides
Basic
7 Non-metal Oxides
Acidic
AlCl3
CuSO4
AlCl3 → Al3+ + 3 ClAl3+ + 6 H2O → Al(H2O)63+
Al(H2O)63+→ Al(H2O)5(OH)2+ + H+ and a weak acid
forms
HCl → H+ + ClCl- is the very weak conjugate base of a strong acid
CuO
Fe2O3
SO2
CO2
CuO + H2O → Cu2+ + 2 OHCopper (II) oxide reacts to form a basic solution.
H2O + SO2 → H2SO3
Sulfur dioxide gas reacts with water to form
sulfurous acid
Problems: Identifying Salt Solutions as Acid, Neutral or Basic:
1.
2.
3.
4.
KCl
K2CO3
NH4NO3
NaF
Find the pH of the following solutions:
1.
2.
3.
4.
5.
6.
0.250 mol/L KCl
0.100 mol/L K2CO3
0.0100 mol/L NH4NO3
0.050 mol/L NaF
0.200 mol/L AlCl3
0.500 mol/L NaC2H3O2
5.
6.
7.
8.
Na2SO3
NH4Cl
AlCl3
N2O
9. NaClO4
10.NH4Cl
11.NaC2H3O2
12.PbO