Acid Base Equilibrium
Problems
1.
Formic acid HCH02, is a monoprotic acid, in a 0.100 M solution of formic acid the pH of the solution was
found to be 2.38 at 25°C. Calculate Ka for formic acid.[Ans Ka = 1.8E-4]
2.
Calculating pH from K3 • The concentration of a sample of vinegar was found to be 0.75M acetic acid,
HC2H30 2. Calculate the pH of this solution. Ka for acetic acid is 1.8 E-5[Ans H+ = 0.0037M, pH = 2.43]
3.
Calculating% ionisation. What is the percentage of acetic acid in 0.10 M HC2H30 2? Ka 1.8E-5[Ans 1.3%]
4.
Write the equilibrium equations and equations for Kb for each of the following Bronsted bases.
a.
CN- ion
b.
C2H 3 0 2- ion
c.
C6 HsNH2- ion
d.
H2 0
5.
Methyl amine, CH 3 NH 2, has a pungent odour being a primary amine. In 0.1000M CH 3 NH 2, the pH is 11.80.
What is the Kb of Methylamine?[Ans Kb= 4.2E-4]
6.
Calculate the values of [OH-], pOH, and pH from Kb and [B initial]. Calculate the values of [OH-], pOH and pH
in a 0.20 M solution of ammonia. Given Kb = 1.8E-5.[Ans = [OH-] = 0.0019, pOH = 2.72, pH= 11.28]
7.
What is the pH of a buffered solution prepared by dissolving O.llM NaC2H3 0 2 and 0.090 M HC2H3 0 2? Ka for
acetic acid is 1.8E-5[H+ = 1.5E-5, pH = 4.82]
8.
Potassium acetate hydrolyzes to give a basic solution. What is the pH of a 0.15 M solution of potassium
acetate? For C2H3 0 2- Kb=5.7E-10.[Ans OH-ions = 9.2E-6, pOH = 5.04, pH= 8.96]
9.
What is the pH of an ammonium salt solution? What is the pH of a 0.10 M solution of ammonium chloride?
Ka of NH/ = 5.7 E-10 [Ans =pH= 5.12]
Acid-Base Unit Questions
1.
Calculate the pH of a solution in which the [H 3 0+] equals:
a) 1.0 M ·
b) 0.1M
c) 0.01M
d) 1.0X 10-7 M
e) 3.0X 10-7 M
f)
4.0X 10-10 M
2.
What is the pOH, and the (H 30+ 1] of a 0.01 M KOH solution?
3.
Calculate the [OH-1 ] of a so~ution if the pH=
a) 11.00
b) 4.68
4.
Calculate the [H 3 0+1 ] of a solution if the pH=
a) 7.89
b) 3.54
5.
Calculate the pH of a solution if it contains:
a) 0.010 mole/L H30+ 1
b) 0.0010 mole/L OH-1
c) 0.50 mole/L H30+1
d) 0.25 mole/L OH-1
e) 0.015 mole/L HN0 3 , assuming HN0 3 is a strong acid.
f) 0.22 mole/L Ba{OHh, assuming Ba(OHh is a strong base.
6.
Determine the pH in each of the following solutions:
a)
lemon juice, [H 3 0+1 ] = 5 X 10-3 mole/L
b)
milk, [H 3 0+1] = 3 X 10-7 mole/L
c)
seawater, [H 3 0+ 1] = 3 X 10-9 mole/L
7.
Benzoic acid, C6 H5COOH, is an organic acid whose sodium salt, C6 H5COONa, has long been used as a safe
food additive to protect beverages and many foods against harmful Y!'!asts and bacteria. The acid is
monoprotic. Write the equation for the Ka of the acid.
8.
What is the percent ionization of a 0.15 M solution of HF? What is the pH of this ionization? {Use Ka value
of the acid)
9.
Periodic acid, HI04, is an important oxidizing agent and a moderately strong acid. In a 0.10 M solution,
[H 30+ 1] = 3.8 X 10-2 moi/L. Calculate the Ka and pK. for periodic acid. [Assume 100% dissociation]
10.
Para-aminobenzoic acid, PABA, is a powerful sun screening agent whose salts are used widely in sun
tanning and screening lotions. The parent acid which we may symbolize as H-Paba, is a weak acid with a
pK. of 4.92 at 25°C. What will be the [H 3 0+1 ] and pH of a 0.030 M solution of this acid?
11.
Ethylamine, CH 3 CH 2NH 2, has a strong pungent odour similar to that of ammonia. Like ammonia, it is a
Bronsted base. A 0.10 M solution has a pH of 11.86. Calculate the Kb and pKb for the ethylamine, and find
the pK. for its conjugate acid, CH 3CH 2 NH 3+1
12.
A sample of blood was found to have a [H 30+ 1] = 4.6 X 10-8 M. Find the molar concentration of OH- 1, and
decide if the sample was acidic, basic, or neutral.
13.
Hydrazine, N2 H4, has been used as a rocket fuel. Like ammonia, it is a Bronsted base. A 0.15 M solution has
a pH of 10.70. What is the Kb and pKb for hydrazine and the pK. of its conjugate acid?
14.
Butanoic acid {butyric acid), C3 H7COOH, is found in small quantities in human perspiration and is
responsible for the foul odour often associated with locker rooms. The acid is also produced when butter
becomes rancid. A 0.0010 mole/L solution of butanoic acid has a pH of 3.91 at 25°C. Calculate the acid
dissociation constant of butanoic acid.
HYDROLYSIS EQUILIBRIUM
Kh
The reaction of an ion with water to produce its parent acid or base is sometimes referred to as
hydrolysis. The equilibrium constant associated with that reaction can be referred to as Kh. However the
ion is acting as an acid or base in the Bronsted sense, and the equilibrium constant may in some books
be referred to as Ka or Kb.
These constants (hydrolysis constants) are not tabulated since they can be calculated using Ka or Kb
values.
HYDROLYSIS:
The salt of weak acids undergoes hydrolysis. The conjugate base of a weak acid is a strong conjugate
base hence hydrolysis is possible as stated in the definition
+
H30+
Conjugate
Acid
+
C2H3o2·
Conjugate
Base
The acetate ions (conjugate base) is a strong conjugate base hence undergoes hydrolysis to make the
solution basic.
C2H302- + H20 <=> HC2H302 + OHDue to the above interaction the aqueous solution becomes richer in OH- ions hence basic.
This can also be achieved by dissolving a strong electrolyte like Na C2H30 2. Being a strong electrolyte
dissociates completely and produces acetate ions which undergo hydrolysis.
As a rule strong conjugate bases always undergo hydrolysis to produce basic solutions.
CATIONS: undergo hydrolysis to form acidic solutions
(A large number of metals ions (cations) usually undergo acid hydrolysis Fe 3+ + 3H 20 <:> Fe(OHh + 3H+)
ANIONS: undergo hydrolysis to form basic solutions
Strong conjugate acids of weak bases easily undergo hydrolysis.
This can also be achieved by dissolving salts of strong conjugate acids eg. NH 4 CI
NH 4 0H <=> NH/ + OHHydrolysis of NH/
NH/ + H20 <=> NH3 + H30+
Due to this change more H+ ions are formed hence makes the aqueous solution acidic.
As a rule strong conjugate acids always undergo hydrolysis to produce acidic solutions
BUFFERS
Buffers are substances (aqueous solutions) that can resist the change of pH on the addition of small
amount of an acid or base.
The purpose of a buffer is to maintain a relatively constant pH. Buffers are usually found in most
biological systems because drastic changes in pH could cause major problems in the different systems of
living organisms.
Example:
If you add 1 ml of 10 moi/L HCI to neutral saline, you change the pH from 7 to 2
If you add 1 mL of 10 moi/L HCI to blood plasma (contains buffers), you change the pH from 7.4 to 7.2
All buffers are weak acids and its conjugate base (salt of conjugate base)
Or Weak base and salt of conjugate acids.
There are two types of buffers
lnvitro = weak acid and weak conjugate base
If you add extra acid
If you add extra base
[H30+]=Kana -noH
nb + noH
In Situ
=strong acid with excess of conjugate base
[H30+]=Ka
n.,·a
nwb- nNsa