Experiment 31
Hydrolysis
Introduction
Most salts are strong electrolytes and exist as ions in aqueous solutions. Many ions react
with water to produce acidic or basic solutions. The reactions of ions with water are frequently
called hydrolysis reactions. We will see that anions such as CN- and C2H3O2- that are derived
from weak acids react with water to form OH- ions, while cations such as NH4+ and Fe3+ that
come from weak bases react with water to form H+ ions.
Hydrolysis of Anions
Let us consider the behavior of anions first. Anions of weak acids react with proton, or
H+, sources. When placed in water these anions react to some extent with water to accept
protons and generate OH- ions, and, thus, cause the solution pH to be greater than 7. Recall that
proton acceptors are Bronsted bases. Thus, the anions of weak acids are basic in two ideas:
They are proton acceptors, and their aqueous solutions have pH’s above 7. The nitrite ion, for
example, reacts with water to increase the concentration of OH- ions:
NO2-(aq) + H2O(1)

 HNO2(aq) + OH-(aq)
This reaction of the nitrite ion is similar to that of weak bases such as NH3 with water:
NH3(aq) + H2O(1)

 NH4+(aq) + OH-(aq)
Thus, both NH3 and NO2- are bases and as such have a basicity or base-dissociation constant, Kb,
associated with their corresponding equilibria.
According to the Bronsted theory, the nitrite ion is the conjugate base of nitrous
acid. Let’s consider the conjugate acid-base pair HNO2 and NO2- and their behavior in water:

HNO2  H+ + NO2NO2- + H2O

 HNO2 + OH-
K=
[H+] [NO2-]
(1a)
[HNO2]
Kb=[HNO2] [OH-]
[NO2-]
(1b)
Multiplication of these dissociation constants yields:
[H+] [NO2-]
[HNO2] [OH-]
Ka x Kb =
[HNO2]
-
[NO2 ]
= [H+] [OH-] = Kw
where Kw is the ion-product constant of water.
Thus the product of the acid-dissociation constant for an acid and the base-dissociation
constant for its conjugate base is the ion-product constant for water:
Ka x Kb = Kw = 1.0 x 10-14 at 25°C
(2)
Knowing the Ka for a weak acid, we can easily find the Kb for the anion of the acid:
Kb =
Kw
Ka
(3a)
By consulting a table of acid-dissociation constants, we find that Ia for nitrous acid is 4.5
x 10-4. Using this value, we can determine Kb for NO2-:
Kb =
1.0 x 10-14
4.5 x 10 -4
= 2.2 x 10-11
We further note that the stronger the acid, that is, the larger the Ka, the weaker its conjugate base.
Similarly, the weaker the acid (the smaller the Ka), the stronger the conjugate base.
Anions derived from the strong acids, such as C1- from HCl, essentially do not react with
water to affect the pH, neither do Br-, I-, NO3-, SO42-, and ClO4- affect the pH, for the same
reason. They are spectator ions in a sense and can be described as neutral ions.
Anions with ionizable protons such as HCO3-, H2PO4-, and HPO42- may be either acidic
or basic, depending on the relative values of Ka and Kb for the ion. We will not consider such
ions in this experiment.
Hydrolysis of Cations
Cations that are derived from the weak bases react with water to increase [H+]; they form
acidic solutions. For example, the ammonium ion is derived from the weak base NH3 and reacts
with water as follows:
NH4+(aq) + H2O(1)

 H3O+(aq) + NH3(aq)
We can represent this acid-dissociation of NH4+ more simply:
NH4+(aq)

 NH3(aq) + H+(aq)
NH3(aq) + H2O(1)

 NH4+(aq) + H-(aq)
Knowing the value of Kb for NH3, we have to calculate the acid dissociation constant
from Equation (3a):
Kw
Ka = K
b
Cations of the alkali metals (Group 1A) and the larger alkaline earth ions, Ca2+, Sr2+, and
Ba2+, do not appreciably react with water, because they come from strong bases. Thus, these
ions have negligible influence on the pH of dilute, aqueous solutions. They are considered as
spectator ions in acid-base reactions. The cations of most other metals do hydrolyze to produce
acidic solutions. Metal cations are typically coordinated with four or six water molecules, and it
is this hydrated ion that serves as the proton donor. The following equations illustrate this
behavior for the iron(III) ion:
Fe(H2O)6 3+(aq) + H2O(1)

 Fe(OH) (H2O)5 2+(aq) + H3O+(aq)
Alternatively, the above reaction may be written as:
Fe3+(aq) + H2O(1)

 Fe(OH)2+(aq) + H+(aq)
Summary of Hydrolysis Behavior
Whether a solution of a salt will be acidic, neutral, or basic, can be predicted on the basis
of the strengths of the acid and base from which the salt was formed.
1.
Salt of a strong acid and a strong base. Examples: NaCl, KBr, and Ba(NO3)2. Neither
the cation nor anion significantly hydrolyzes, and the solution has a pH of 7.
2.
Salt of a strong acid and a weak base. Examples: NH4Br, ZnCl2, and Al(NO3)3. The
cation hydrolyzes, forming H+ ions, and the solution has a pH less than 7.
3.
Salt of a weak acid and a strong base. Examples: NaNO2, KC2H3O2, and Ca(OC1)2.
The anion hydrolyzes, forming OH- ions, and the solution has a pH greater than 7.
4.
Salt of a weak acid and a weak base. Examples: NH4F, NH4C2H3O2, and Zn(NO2)2.
Both ions hydrolyze, the pH of the solution is determined by the relative extent to which
each ion hydrolyzes.
Procedures
Measure the pH of water and of several available 0.1 M aqueous salt solutions to
determine whether these solutions are acidic, basic, or neutral. Write the hydrolysis reactions for
CuCl2, NH4Br, K2CO3, and Na3PO4.
Calculations
Use equation (1a) and your data to calculate a Ka for the NH4Br hydrolysis. Then use
equation (1b) and your data to calculate a Kb for the K2CO3 hydrolysis.
Use of Litmus
The chart below shows what is observed when an acidic, basic, or neutral substance is
tested with litmus paper.
Red litmus paper
Blue litmus paper
Acid
stays red
turns red
Base
turns blue
stays blue
Neutral
stays red
stays blue
Note that both red and blue litmus paper must be used to identify neutral substances. Also,
litmus paper should be moistened with water before it is used to check the acidity of a nonaqueous substance.
Lab ________________
Hydrolysis Table
Which
ion
will
Will OH- or
Salt
Anion Cation
+
hydrolyze
NaCl
sodium
chloride
Net ionic equation if there is hydrolysis
CuCl2
Cupric
chloride
Net ionic equation if there is hydrolysis
K2CO3
Potassium
carbonate
Net ionic equation if there is hydrolysis
NH4Cl
Ammonium
chloride
Net ionic equation if there is hydrolysis
NaC2H3O2
Sodium
acetate
Net ionic equation if there is hydrolysis
KNO3
Potassium
nitrate
Net ionic equation if there is hydrolysis
Na3PO4
Sodium
phosphate
Net ionic equation if there is hydrolysis
H3O be in
excess?
Name_________________
Observed pH Calculate Ka or Kb
if there is one
Lab __________________
Chem 1B
Lab Handout
Acids & Bases (II)
Name_____________
1. The Ka for acetic acid(CH3COOH) is 1.81x10-5.
a. Calculate the hydrogen ion concentration of a 0.55-M acetic acid solution
_________
b. Calculate the pH of this solution.
_________
2. Nitrous acid, HNO2, is a weak acid with a Ka of 4.5x10-4.
a. Calculate the H+ concentration of 0.75-M solution of nitrous acid.
________
b. Calculate the pH of this nitrous acid solution
_________
3. a. What is the OH- concentration of a 3.00 M solution of ammonia. The Kb for ammonia
is 1.8x10-5.
_______
b. Calculate the pH of this ammonia solution.
_______
4. Calculate the pH of a 0.15 M solution of sodium acetate (CH3COONa). If the Kb for this
base is 5.6x10-10.
_______________
5. Calculate the pH of a 0.24M sodium formate solution.(HCOONa) (Kb=5.9x10-11)
_______________