8.6: ACID-BASE PROPERTIES OF SALT SOLUTIONS

When some salts dissolve in water, they dissociate into hydrated ions that may or may not affect
the pH of the solution.

They form individual hydrated ions, called hydrolysis of ions or salt

Salts like sodium chloride, which are strong electrolytes dissociate completely into aqueous
sodium and chloride ions without upsetting the [H+] and [OH-] of the water.

The pH of the solution is 7

On the other hand, when sodium carbonate, another highly soluble salt dissociate in water, it
increases the pH of the solution (basic solution).

This property must be explained by the carbonate ions present,because both salts contain sodium
ions in solution.

The basic character of the carbonate can be explained by the Bronsted-Lowry theory as seen
below.

In general, since salts have two different ions, the pH of aqueous salt solution may be affected by
the cations, anions, or both
Therefore, hydrolysis is the reaction of an ion with water to produce an acidic or basic solution.
PREDICTING IF A SALT WILL PRODUCE A NEUTRAL, AN ACIDIC OR BASIC
SOLUTION
1 Salts that form Neutral solutions
Salts from cations of strong bases e.g., NaOH, KOH,
Na+(aq) and K+(aq) does not react with water to form Hydroxide and hydrogen ions
Salts of anions of strong acids e.g., HCl, HI, HNO3, HBr, HCIO4
CI-, NO3-, Br-, CIO4-; will not form hydroxide with water.
2. Salts that form acidic solutions.
They hydrolyze to form hydronium ions in solution (act as Bronsted-Lowry acids)
Composition and hydrolysis of Acidic salts
Salts
Cation
Hydrolysis reaction
NH4CI NH4+ (aq) NH3
NH4+ (aq) + H2O(l)↔ H3O+(aq) + NH3 (aq)
(acid)
weak base
Salts
Cation
N2H5Br N2H5+ (aq)
(a(acid)cid
N2H4
weak base
Hydrolysis reaction
N2H5+ (aq) + H2O (l) ↔ H3O+ (aq) + N2H4(aq)
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
Cations like Al3+ (aq), Fe3+ (aq) and Sn2+(aq) (transition metals) can hydrolyze and act as acid, they
have large positive densities (a large charge in small volume).

When they become hydrated, their highly charged density enables the attached water molecule to
donate a proton to the water creating hydronium.
Al3+ (aq) + 6H2O (l) ↔ Al(H2O)63+ (aq)
__H+(aq)______
↑
↓
Al(H2O)63+ (aq) + H2O (l) ↔ H3O+ (aq) + Al(H2O)5(OH)2+ (aq)
Acid
Base
Conj. acid Conj. Base
Determining the pH of an acidic Salt Solution
Sample question 1:
Calculate the pH of a 0.525 mol/L solution of ammonium chloride, NH4Cl(aq). (The Kb value for ammonia,
NH3(aq) is 1.8 x 10-5) (ans: 4.77)
3. Salts that form basic solutions

Sodium chloride solution is neutral, but sodium acetate is basic. The anion of a strong acid is too
weak a base to affect the pH of an aqueous solution (e.g., CI- (aq) and NO3- (aq))

The anion of weak acid, although it is too weak, is strong enough base to affect the pH of an
aqueous solution by increasing the pH and making it more basic.(e.g., potassium ethanoate,
KC2H3O2)
Determining the pH of an Basic Salt Solution
Sample question 2:
Calculate the pH of a 0.35 mol/L solution of sodium methanoate, NaCHO2(aq). (The Ka methanoic acid,
HCO2(aq) is 1.8 x 10-4) (ans: 8.64)
4. Salts that act as acids and bases.
Some salt contain the cation of a weak base and the anion of a weak acid. Both ions can hydrolyze
e.g., NH4CN (s)
NH4CN (s) → NH4+ (aq) + CN-(aq)
(dissociation)
NH4+ (aq) + H2O (aq) ↔ NH3(aq) + H3O+ (aq) Ka = 5.8 x 10-10 mol/L
CN- (aq) + H2O (aq) ↔ HCN (aq) + OH- (aq)
Kb = 1.6 x 10-5 mol/L
Since the Kb of the cyanide ion (1.6 x 10-5 mol/L) is much higher than the Ka of ammonium ion (5.8 x 1010
), an aqueous solution of ammonium cyanide will be basic.
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HYDROLYSIS OF AMPHOTERIC IONS

All polyatomic ions whose chemical formulas start with hydrogen,
e.g., HCO3-(aq) and HSO4- (aq) are amphoteric.

When NaHCO3 (aq) dissolves in water, the dissociation is as follows:
NaHCO3 (s) → Na + (aq) + HCO3- (aq)

Since it is amphoteric, the HCO3- (aq) may hydrolyze to be an acid or a base according to the
equation:
HCO3- (aq) + H2O (l) ↔ H3O +aq) + CO32- (aq) (Acid hydrolysis)
HCO3- (aq) + H2O (l) ↔ OH- (aq) + H2CO3 (aq) (Base hydrolysis)

However, one equilibrium predominates and gives rise to the overall acidic or a basic character of
the aqueous solution.
Example:
Predict whether a solution of NaHBO3 (aq) is acidic, basic, or neutral.
Solution:
NaH2BO3 (aq) → Na+ (aq) + H2BO3- (aq)
(dissociation)
H2BO3- (aq) + H2O (l) ↔ HBO32- + H3O+(aq)
Ka = 5.8 x 10-10 (acid hydrolysis)
H2BO32- (aq) + H2O (l) ↔ H3BO3 (aq) + OH- (aq) Kb = 1.7 x10 -5
(base hydrolysis)
since Kb > Ka, a solution of NaH2BO3 (aq) is basic
HYDROLYSIS OF METAL AND NONMETAL OXIDE
1. Metal oxides react with water to produce basic solutions
e.g., CaO (s) + H2O (l) → Ca2+ (aq) + 2OH- (aq)
Hydroxide is produced according to the equation:
O2- (s) + H2O (l) → 2OH- (aq)
2. Nonmetal oxides react with water to produce acidic solutions.
e.g., CO2 (g) + H2O (l) ↔ H2CO3 (aq)
H2CO3 (aq) + H2O (l) ↔ H3O+ (aq) + HCO3- (aq)
CO2 (g) + 2H2O (l) ↔ H3O+ (aq) + HCO3- (aq)
( net equation)
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