Chemistry
Acids & Bases
SALT SOLUTIONS
Salts
Salts are simply ionic compounds. They are the result of a strong acid and a strong base (i.e. acid-base
neutralization reaction). Salts can be acidic, basic, or neutral in an aqueous environment.
Overview
Strong Acid
Strong Acid
weak acid
weak acid
+
+
+
+
Strong Base
weak base
Strong Base
weak base




neutral salt
acidic salt
basic salt
Ka > Kb = acidic salt
Kb > Ka = basic salt
Strong Acid + Strong Base:
HCl + NaOH
H+ + Cl– + Na+ + OH–  NaOH + H2O
NaOH is a neutral salt because at equilibrium the reaction is completely shifted to the right.
Strong Acid + weak base
HCl + NH3
H+ + Cl– + NH3
+
The ammonia will react with the H :
H+ + NH3 + Cl–
NH4Cl
+
NH3 is a weak base making NH4 a relatively strong conjugate acid. The ammonium ion (a relatively
+
+
strong conjugate acid) will then react with water to produce:
NH4 + H2O
NH3 + H3O
Thus making an NH4Cl an acidic salt.
H+ + C2H3O2– + Na+ + OH–
Na+ + C2H3O2– + H2O
–
Because HC2H3O2 is a weak acid, the basic conjugate form predominates (C2H3O2 ). Because water is
the predominate species in an aqueous environment, the basic salt is formed from the hydrolysis reaction:
–
–
C2H3O2 + H2O
HC2H3O2 + OH
–
It is the formation of the OH that makes NaC2H3O2 a basic salt.
weak acid + Strong Base:
HC2H3O2 + NaOH
weak acid + weak base
HF + NH3
NH4F + H2O
NH4+ + F–
If the Ka of NH4+ (tendency for NH3 to form NH4+) is stronger than the Kb for F– (tendency to form HF),
then the reaction will proceed to the right: NH4F is an acidic salt (NH4+ + H2O
NH3 + H3O+).
–
Conversely, if Kb of F (tendency to for HF) is stronger, then the reaction will proceed to the left: NH4F
is a basic salt (F– + H2O
HF + OH–). Thus, there is a competition between Ka and Kb.
–10
KaNH4 = 5.6 x 10
KbF = 1.4 x 10–11
Ka > Kb so NH4F is an acid salt (pH ~6.2)
Helpful



Group 1 cations & heavier Group 2 (Ca2+, Sr2+, Ba2+) and Mg2+ are neutral = spectator ions.
Transition metals & Al3+ are slightly acidic.
Strong acids produce neutral cations. Strong bases produce neutral anions. Both drive the
reactions to the far left. (e.g., HCl  H+ + Cl–)
Summary
Salt
NH4I
Na3PO4
KClO4
Cation
+
NH4 (acidic)
+
Na (spectator)
+
K
(spectator)
Anion
–
I
PO4
–
ClO4
(spectator)
(basic)
(spectator)
Salt
acidic
basic
neutral
E:\2014-2015\340_Masterton\340_Sections\340.13_Acids_and_Bases\340.14.08.Salt Acid-Base Properties .docx; Masterton
Ch. 13: Acids & Bases
Chemistry
Strong Acids
Strong Bases
HCl
HNO3
LiOH
Ca(OH)2
HBr
HClO4
NaOH
Sr(OH)2
H2SO4
HI
(note: halogen but not HF,
H3PO4 are weak acids)
Ba(OH)2
KOH
(group 1 & heavy group 2)
77. Write formulas for two salts that:
3+
Ni+ is slightly acidic; want spectator anion (from
a. contain Ni
and are acidic strong acid)
b. contain Na+
and are basic
–
c. contain ClO4
and are neutral
+
d. contain NH4
and are acidic
Na+ is neutral; want basic anion (from weak acid)
2–
c. Br– and are
neutral
d. contain ClO4
and are acidic
–
+
NH4 is acidic; want spectator anion (strong acid)
2–
Because CO3 is a basic salt (comes from a weak
acid HCO3–), want neutral (spectator) ion
–
Br is neutral (strong acid); want neutral (spectator)
e.g., cation from strong base or group or heavier
group 2
–
1. NaF
2. Na3PO4
ClO4 is a spectator ion; want spectator cation (from 1. NaClO4
strong base or neutral ion)
2. KClO4
78. We may need to do some of these using a Ka/Kb table (see text)
+
+
a. NH4 and are
NH4 Ka = 5.6 x 10–10; want Kb > Ka
basic
(e.g., anion from weak acid)
–4
Kb ClO– = 3.6 x 10–7; Kb CO32– = 2.1 x 10
b. CO3 and are
basic
1. NiCl3
2. Ni(NO3)3
–
ClO4 is neutral (strong acid); want acidic cation
3+
2+
(eg., transition, Al , or to a lesser extent, Mg
1. NH4Cl
2. NH4NO3
1. NH4ClO
2. NH4CO3
1. Na2CO3
2. MgCO3
1. NaBr
2. MgBr2
1. Al(ClO4)3
2. Sr(ClO4)2
Chemistry
79. State whether 1 M solution of the following salts in water would be acidic, basic, or neutral.
+
2–
basic salt
a. K2CO3
K = spector; CO3 = basic
–
+
slightly acidic salt
(Ka > Kb)
b. NH4F
NH4 = slightly acidic; F = slightly basic
–10
–11
pKa = 5.6 x 10 ;
Kb = 1.4 x 10 ;
c. LiH2PO4
Li = spectator; H2PO4 = ?
–
for H2PO4 :
–
2–
+
H2PO4 (aq) + H2O(l)
HPO4 (aq) + H3O (aq)
or
–
–
H2PO4 (aq) + H2O(l)
H3PO4(aq) + OH (aq)
–
+
+
–12
basic
–
neutral
Na = spectator;
NO2 = basic
e. Ba(ClO4)2
Ba = spectator; ClO4 = spectator
(both come from strong)
–8
Kb = 1.4 x 10
Ka > Kb  acidic
–
d. NaNO2
2+
Ka = 6.2 x 10
81. Write the net ionic equations to explain the acidity or basicity of the various salts in #79.
+
a. K2CO3 (basic) K is spectator, so doesn’t participate in reaction.
2–
CO3 is basic and makes the salt basic.
(Remember: aqueous solutions are composed mostly of water. Even in
strong acids and strong bases.)
2–
CO3 (aq) + H2O(l)
b. NH4F (acidic)
–
–
HCO3 (aq) + OH (aq)
–
(the produced OH makes the salt basic)
+
NH3(aq) + H+(aq)
or
–
–
F (aq) + H2O(l)
HF(aq) + OH (aq)
NH4 (aq)
Ka = 5.6 x 10
–10
Kb = 1.5 x 10
–11
Ka > Kb  acidic
c. LiH2PO4
(acidic)
–
2–
H2PO4 (aq) + H2O(l)
–
H2PO4 (aq) + H2O(l)
+
HPO4 (aq) + H3O (aq)
or
–
H3PO4(aq) + OH (aq)
e. Ba(ClO4)2
+
(Na is spectator)
–
NO2 (aq) + H2O(l)
2+
–8
Kb = 1.4 x 10
–12
Ka > Kb  acidic
+
(Li is spectator)
d. NaNO2 (basic)
Ka = 6.2 x 10
basic
–
HNO3(aq) + OH (aq)
(Ba is heavy group 2  spectator)
–
ClO4 (comes from strong acid = spectator ion)
(neutral salt)
Chemistry
83. Arrange the following 0.1 M aqueous solutions in order of decreasing pH (highest to lowest)
Remember: Strong or weak = amount of dissociation. This differs from the concentration
of an acid or base, which refers to molarity. That’s why question makes concentrations the
same.
Compare Ka’s and Kb’s.
NaF
Ba(NO3)3
HNO3
NH4NO3
Al(NO3)3
1. Highest pH will be from the strongest acid. HNO3 is the only strong acid.
#1 = HNO3
2. F– is the conjugate base of a weak acid ( relatively strong base) (Na+ = spectator)
#5 = NaF
+
NH4 is strong conjugate acid of weak base;
3+
Al = weak acid (comparable to transition metals)
1 – NaF
spectator + weak base
2 – Ba(NO3)2
spectator + spectator
3 – NH4NO3
weak acid (Ka = 5.6 x 10
4 – Al(NO3)3
weak acid (aqueous as Al(H2O6) ; Ka = 1.2 x10 ) + spectator
5 – HNO3
strong acid
–10
) + spectator
3+
–5