EXPERIMENT 17:
REACTIONS OF SALTS WITH WATER
INTRODUCTION
A. HYDROLYSIS
Hydrolysis, as applied to aqueous solutions of inorganic compounds, can be defined as the reaction
of water with one or both ions of a salt to form a weak acid and OH , or a weak base and H 3O+, or
both.
Example 1: In an aqueous solution of NaCN the solute particles are Na + and CN . The CN  ion is
the conjugate base of a weak acid and it reacts with water as follows:
CN(aq) + H 2O(l) 
HCN(aq) + OH (aq)
The resulting solution is basic.
Example 2: In an aqueous NH 4Cl solution the solute particles are NH 4+ and Cl . The ammonium
ion, NH 4+, is the conjugate acid of a weak base and reacts with water to give an acidic solution
as follows:
NH4+(aq) + H2O(l)  H3O+(aq) + NH 3(aq)
Example 3: The solute particles in an aqueous NaCl solution are Na + and Cl, neither of which
react with water. Therefore an aqueous solution of NaCl is neutral.
Example 4: The solute particles in an aqueous solution of NH 4C2H3O2 are NH4+, which is the
conjugate acid of a weak base, and C 2H3O2, which is the conjugate base of a weak acid. Both
ions react with water:
NH4+(aq) + H2O(l)  H3O+(aq) + NH 3(aq)
C2H3O2(aq)
+ H 2O(l)

HC2H3O2 (aq) + OH (aq)
B. EQUILIBRIUM CONSTANT FOR HYDROLYSIS REACTION
Consider the reaction that occurs in a solution of NaC 2H3O2.
C2H3O2(aq)
+ H 2O(l)

HC2H3O2(aq) + OH (aq)
The equilibrium constant expression for this reaction is
[HC2H3O2][OH]
Kb =
[C 2H3O2]
Unfortunately, there is no table that will have a value for
a Kb for C2H3O2. Therefore the K b will have to be
calculated as follows:
Kb =
Derivation of K b for C 2H3O2:
Kb = [HC2H3O2][OH]
[C 2H3O2]
Kb =
K w_
K a of HC2H3O2
K b = 1.0 x10 –14 = 5.9 x 10 -10
1.7 x 10 -5
Kb =
[H3O+]
[H3O+]
[HC2H3O2]
[OH][H3O+]
+
[C 2H3O2][H3O ]
Kw
1/Ka of HC2H3O2
Kw_
K a of HC2H3O2
1
Consider the reaction that occurs in a solution of NH 4Cl:
NH4+(aq)
+ H2O(l) 
H3O+(aq) + NH 3(aq)
The equilibrium constant expression for this reaction is
Ka =
Derivation of K a for NH4+
[NH3][ H3O+]
[NH4+]
Ka =
Unfortunately, there is no table that will have a value for
a Ka for C2H3O2. Therefore the K a will have to be
calculated as follows:
Ka =
Ka =
Kw
K b of NH3
[NH3][ H 3O+]
[NH 4+]
[NH3]
[OH]
[NH 4+]
[OH]
[OH]
[H3O+][OH]
1/Kb of NH3
K a = 1.0 x10 –14 = 5.6 x 10 -10
1.8 x 10 -5
Ka =
Kw
Kw
K b of NH3
From the examples above you notice that the weaker the acid or base produced in the reaction, the
greater the extent of hydrolysis. (The weaker the acid, the stronger its conjugate base; and the
weaker the base, the stronger its conjugate acid.)
C. SIMULTANEOUS HYDROLYSIS BY BOTH THE CATION AND ANION OF A SALT
If both ions of a salt hydrolyze, the extent of hydro lysis will be greater than it would be for either
ion by itself. Consider an aqueous solution of NH 4C2H3O2. Both ions hydrolyze as follows:
NH 4+(aq)
C2H3O2(aq)
The
+ H2O(l) 
+ H2O(l)

H3O+(aq) + NH3(aq)
K a = 5.6 x 10 -10
HC2H3O2(aq) + OH (aq)
K b = 5.9 X 10 -10
H3O+produced in the first reaction reacts with the OH produced in the second reaction,
H3O+(aq) + OH (aq)
 H2O(l)
thereby shifting both equilibrium reactions to t he right.
Whether the solution of NH4C2H3O2 will be acidic or basic can be determined by comparing the
equilibrium constants for the reactions. The reaction with the higher K will be the reaction that
predominates. In the case of NH4C2H3O2, the solution will be basic since K b  Ka.
2
D. HYDROLYSIS BY AN ACID SALT
In most cases, when it is the cation of a salt that hydrolyzes, that cation (for example, NH4+) is the
conjugate acid of a weak base, so it acts as an acid in solution. And usually, when it is the anion of
a salt that hydrolyzes, that anion (for example, C2H3O2) is the conjugate base of a weak acid, so it
acts as a base in solution. However, some anions can act as acids in solution as well. Some
examples are HCO 3, HSO3, H2PO4, and so on. These anions are amphoteric —they can act as
both acid and base. For example, in a solution of sodium bicarbonate the HCO 3 ion reacts as
follows:
HCO3(aq) +
H 2O(l) 
HCO3(aq) + H 2O(l) 
H3O+ (aq) + CO 32(aq)
H2CO3(aq) + OH (aq)
Ka2 = 4.8 X 10 -11
Kb = 1 X 10 -14
4.3 x 10 -7
= 2.3 X 10 -8
( Ka1 of H2CO3)
To determine which reaction predominates, compare the equilibrium constants for th e two
reactions. In the case of HCO 3, Kb > Ka2, therefore a sodium bicarbonate solution will be basic.
(Note that a salt containing HSO4  would be called an acid salt because HSO 4 does act as an acid,
however, HSO 4 is not amphoteric—it cannot also act as a base as do HCO 3, HSO3, H2PO4, and
so on, because it is the conjugate base of a strong acid.)
EXPERIMENT
Check out a pH pen from the stockroom.
A. RELATIVE DEGREE OF HYDROLYSIS OF VARIOUS SALTS
Place 0.10 M solutions of each of the following in separate wells of a spot plate.
ammonium acetate
ammonium chloride
ferric chloride
sodium acetate
sodium bicarbonate
sodium bisulfate
sodium carbonate
sodium chloride
sodium dihydrogen phosphate
NH4C2H3O2
NH4Cl
FeCl3
NaC2H3O2
NaHCO3
NaHSO4
Na2CO3
NaCl
NaH2PO4
Use the pH pen to measure the pH of each solution and record on your Report Sheet. Remember to
rinse the tip of the pH pen with tap water between tests. Record the results on your Report Sheet
and answer the questions.
3
B. HYDROLYSIS OF AMMONIUM SALTS
Remove the stoppers and cautiously smell the odors from bottles of solid ammonium acetate, solid
ammonium carbonate, and solid ammonium chloride, from the Chem 111 shelves. (There is enough
water adsorbed on the surface of the apparently dry crystals to make hydrolysis possible).
Decide which salt has the strongest ammonia odor and which has the weakest ammonia odor. Write
your results on the Report Sheet and answer the questions.
C. HYDROLYSIS OF Al 3+ ION
1.
Mix about 1 g of dry Al 2(SO4)3 and 1 g of dry NaHCO 3. Write your observations on your Report
Sheet and answer the questions that follow.
2.
Add a few milliliters of deionized water to the mixture of Al 2(SO4)3 and NaHCO 3 prepared in step
1, above. Write your observations on page 9 of your Report Sheet and answer the questions
that follow.
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REPORT SHEET
Experiment 17: Hydrolysis
Name ________________________________________
last
first
Instructor’s Initials ________
A. RELATIVE DEGREE OF HYDROLYSIS OF VARIOUS SALTS
DATA
Measured pH
TABLE 1
ammonium acetate
NH4C2H3O2
ammonium chloride
NH4Cl
ferric chloride
FeCl3
pOH
CALCULATIONS
[H3O+]
[OH]
NaC2H3O2
sodium acetate
sodium bicarbonate
NaHCO3
sodium bisulfate
NaHSO4
sodium carbonate
Na2CO3
NaCl
sodium chloride
sodium dihydrogen phosphate
NaH2PO4
In Table 2 below, write the net ionic equation for the ionization reaction(s) with water for each salt
in Table 1 above.
TABLE 2
SALT
EQUATION
NH4C2H3O2
NH4Cl
FeCl3
Fe3+ + 2 H2O  FeOH2+ + H3O+
NaC2H3O2
NaHCO3
NaHSO4
Na2CO3
NaCl
NaH2PO4
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1.
Percent hydrolysis of sodium acetate in 0.1 M NaC2H3O2 solution.
a.
Copy the net ionic equation for the reaction of NaC 2H3O2 and water from Table 2.
__________________________________________________________________________
2.
b.
Write the equilibrium constant expression for the above reaction.
c.
Calculate the value of K b for the reaction (K a for HC2H3O2 = 1.7 x 10 -5)
d.
From K b calculated above and initial concentration of NaC 2H3O2 (0.10 M), calculate the
theoretical hydroxide ion concentr ation at equilibrium.
e.
Calculate the theoretical percent hydrolysis of NaC 2H3O2.
f.
Calculate experimental % hydrolysis (use the [OH ] for the NaC 2H3O2 reported in Table 1.)
Percent hydrolysis of sodium carbonate in 0.10 M Na2CO3 solution.
a.
Copy the net ionic equation for the reaction of Na 2CO3 and water from Table 2.
__________________________________________________________________________
b.
Write the equilibrium constant expression for the above reaction.
c.
Calculate the value of Kb for the reaction (K a for HCO 3 = 4.8 x 10 -11).
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3.
d.
From Kb calculated above and initial concentration of Na2CO3 (0.10 M), calculate the theoretical
hydroxide ion concentration at equilibrium.
e.
Calculate the theoretical percent hydr olysis of Na2CO3.
f.
Calculate experimental % hydrolysis (use the [OH ] for the Na2CO3 reported in Table 1.)
Comparison of percent hydrolysis in 0.10 M aqueous solutions of NaC2H3O2 and Na2CO3
a.
Theoretical % hydrolysis of NaC2H3O2 from part 1 e on previous page.
________________
b.
Theoretical % hydrolysis of Na2CO3 from part 2 e above.
________________
c.
Which hydrolyzes more, C2H3O2 or CO32?
________________
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B. HYDROLYSIS OF SOLID AMMONIUM SALTS
1.
2.
Rank the three salts, NH 4C2H3O2, (NH4)2CO3, or NH4Cl, in the order of strength of ammonia
odor:
a.
Strongest ammonia odor
____________________
b.
Medium ammonia odor
____________________
c.
Weakest ammonia odor?
____________________
Odor strength explained in terms of K and % hydrolysis
a.
Solute particles in solution of salt with the strongest ammonia odor: ________________
(1) Cation hydrolysis
Net ionic equation for the reaction of the cation with wa ter (copy from page 2):
______________________________________________________________________
Ka value: (copy from page 2):
_______________________
(2) Anion hydrolysis
Net ionic equation for the reaction of the anion with water :
______________________________________________________________________
Kb value:
b.
_______________________
Solute particles in solution of salt with the medium ammonia odor: __________________
(1) Cation hydrolysis
Net ionic equation for the re action of the cation with water :
______________________________________________________________________
Ka value:
_______________________
(2) Anion hydrolysis
Net ionic equation for the r eaction of the anion with water:
______________________________________________________________________
Kb value:
c.
_______________________
Solute particles in solution of salt with the weakest ammonia odor: __________________
Net ionic equation for the reaction with water:
___________________________________________ _______________________________
Ka value:
_______________________
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3.
Which salt hydrolyzed the least?
_______________________
4.
Compare the extent of hydrolysis of NH4C2H3O2 and (NH4)2CO3 by comparing the K b and %
hydrolysis values for C2H3O2 and CO32.
Kb
C2H3O2
CO32
a.
From the Kbs of the above ions and the odor of their ammonium salts, which would you say
is hydrolyzing to a greater extent?
_______________________
c. Explain the effect of the extent of hydrolysis of the anion on the extent of hydrolysis of the
ammonium ion.
_____________________________________________ _____________________________
C. HYDROLYSIS OF Al 3+ ION
1.
When you mix dry Al 2(SO4)3 and dry NaHCO 3, does a reaction occur? __________________
2.
When you add water to the mixture of Al 2(SO4)3 and NaHCO 3, does a reaction occur? ____
Observations: _______________________________________________________________
3.
Write the net ionic equation for t he reaction of Al 3+ and water
Al3+ + 2 H2O  AlOH2+ + H3O+
Would the solution acidic or basic?
4.
__________________
Write the net ionic equation for the reaction that occurs between the H 3O+ produced in the
reaction in #3 above and the bicarbonate ion (from NaHCO3) .
__________________________________________________________________________
5.
Write the net ionic equation for the reaction that produced the gas.
________________________________ __________________________________________
6.
Add the three net ionic equations above. Drop out terms that appear on both sides. The result
is the net ionic equation for the overall reaction.
_______________________________________________________ ___________________
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EXERCISES
1. You have tested experimentally the pH of 0.10 M NaH2PO4 solution.
2.
a.
Predict whether the solution is (theoretically) acidic or basic. Be sure to write the net ionic
equations for the two reactions that occur. Give the setups for all calculations you make.
(Look up relevant equilibrium constants.)
b.
What was the measured pH of the 0.10 M NaH2PO4 solution?
__________
Was the solution acidic or basi c?
__________
Predict whether an aqueous solution of ammonium bisulfide would be acidic, basic, or neutral. Write
the net ionic equations for the reactions that occur. Give the setups for all calculations you make.
(Look up relevant equilibrium constants.)
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3.
For each of the salts below, predict whether its aqueous solution would be acidic, basic or neutral.
Write the net ionic equation for any reaction that occurs.
COMPOUND
ACIDIC, BASIC,
OR NEUTRAL
NET IONIC EQUATION
Ca(HSO4)2
KNO2
Na2CO3
LiBr
Na3PO4
Al(NO3)3
K2 S
Na2SO4
KNO3
FeCl3
NH4Cl
KCl
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4.
Which salt would hydrolyze more in aqueous solution, KF or KOCl? Write the net ionic equations for
the reactions that occur and give the setups for all calculations made.
(Look up relevant equilibrium constants.)
5.
Which salt would hydrolyze more in aqueous solution, NH 4OCl or NaOCl? Write the net ionic
equations for the reactions that occur and give the setups for all calculations made. (Look up
relevant K values on the table of equilibrium constants.) Explain your answer.
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EQUILIBRIUM CONSTANTS
Acid-Ionization Constants, K a, at 25°C.
Substance
Acetic acid
Benzoic acid
Boric acid
Carbonic acid
Formula
HC2H302
HC7H502
H3B03
H2C03
HC03
Chlorous acid
Cyanic acid
Formic acid
Hydrocyanic acid
Hydrofluoric acid
Hydrogen sulfate ion
Hydrosulfuric acid
HClO2
HOCN
HCH02
HCN
HF
HS04
H 2S
HS
HBrO
HClO
HN02
H 2 C2 0 4
HC204
H3P04
H2P04
HPO42
Hypobromous acid
Hypochlorous acid
Nitrous acid
Oxalic acid
Phosphoric acid
Phosphorous acid
Ka
H3P03
H2P03
HC3H502
HC3H303
H2SO4
HSO4
Propionic acid
Pyruvic acid
Sulfuric acid
Sulfurous acid
H2S03
HS03
1.7  10-5
6.3  10-5
5.9  10-10
4.3  10-7
4.8  10-11
1.1 x 10 -2
3.5  10-4
1.7  10-4
4.9  10-10
6.8  10-4
1.1  10-2
8.9  10-8
1.2  10-13
2.1 x 10 -9
3.5  10-8
4.5  10-4
5.6  10-2
5.1  10-5
6.9  10-3
6.2  10-8
4.8  10-13
1.6  10-2
7  10-7
1.3  10-5
1.4  10-4
strong
1.3 x 10 -2
1.3  10-2
6.3  10-8
Base Ionization Constants, K b, at 25°C.
Substance
Formula
Kb
Ammonia
NH3
1.8  10-5
Aniline
C6H5NH2
4.2  10-10
Dimethylamine
(CH3)2NH
5.1  10-4
Ethylamine
C2H5NH2
4.7  10-4
Hydrazine
N 2H 4
1.7  10-6
Hydroxylamine
NH20H
1.1  10-8
Methylamine
CH3NH2
4.4  10-4
Pyridine
C5 H 5 N
1.4  10-9
Urea
NH2CONH2
1.5  10-14
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