BUFFER SOLUTION

7. Ionic Equilibria MADAM SUHAILA, CHEMISTRY 1

Chapter 7: IONIC EQUILIBRIA

7.1 ACIDS & BASES (PART 2)

a) Define acid and base according to Arrhenius and Bronsted Lowry theories. C1
b) Identify conjugate acid and conjugate base according to Bronsted Lowry theory. C2
c) Relate pH and pOH to the ionic product of water, Kw at 25oC. C2
d) Define strong acid and base, weak acid and base, pH, pOH and pKw. C1
e) Calculate the pH values of a strong acid and base. C2
f) Relate the strength of a weak acid and a weak base to the respective dissociation constants, Ka and Kb. C3
g) Perform calculations involving C3

i) pH
ii) dissociation constant
iii) initial concentration and equilibrium concentration
iv) degree of dissociation, 
h) Explain qualitatively salt hydrolysis using hydrolysis equations and classify the salts formed from the reaction between: C2
i) strong acid and strong base
ii) strong acid and weak base
iii) weak acid and strong base
i) Define buffer solution C1
j) Describe how a buffer solution controls its pH, C2, C3
k) Write the Henderson Hasselbalch equation for a given buffer solution. C1
l) Calculate the pH of buffer solutions using the Henderson Hasselbalch equation before and after the addition of strong acid or
base. C3, C4

A. BUFFER SOLUTION

Buffer solution
is a solution which has the ability to maintain its pH when
a small amount of strong acid or strong base is added to the solution.

It must contain:  Acidic component – that can react with base added [OH–]
 Basic component – that can react with acid added [H+]

Two types of buffer solution
1. Acidic Buffer – contains weak acid and its conjugate base (pH < 7)

Example:

A weak acid and its conjugate base

Mixture of CH3COOH / CH3COONa

Acidic component : CH3COOH
Basic component : CH3COO-

2. Basic Buffer – contains weak base and its conjugate acid (pH > 7)
Example:
A weak base and its conjugate acid
Mixture of NH3 / NH4Cl

Acidic component : NH4+
Basic component : NH3

Buffer solution contains only either weak acid or weak base. If the system contains strong acid
or base, the system cannot act as a buffer solution.

Which of the following pairs is a buffer system?
a) HCN and NaCN
b) HBr and KBr
c) NH3 and (NH4)2SO3
d) NaOH and NaCl

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7. Ionic Equilibria MADAM SUHAILA, CHEMISTRY 2

B. HOW ACIDIC BUFFER MAINTAIN IT’S pH?

Acidic Buffer: CH3COOH / CH3COONa

➢ CH3COOH – weak acid CH3COO- (aq) + H+ (aq)
Undergoes partial dissociation
Dissociation equation: CH3COOH (aq)

➢ CH3COONa – conjugate base (salt) CH3COOH
Undergoes completely dissociation CH3COO-
Dissociation equation: CH3COONa (aq) → CH3COO- (aq) + Na+ (aq)
CH3COO-
CH3COOH

Case #1: When a small amount of strong acid (H+) is added

H+ CH3COO- (conjugate base) in buffer solution reacts with the H+ ion added

CH3COOH CH3COO- (aq) + H+(aq) → CH3COOH (aq)
CH3COO-
Acid (H+) added is completely consumed. concentrations of
CH3COO- Concentration of [CH3COOH] increase, but
CH3COOH [CH3COO-] decrease.
As a result, pH of the solution is not much affected.

Case #2: When a small amount of strong base (OH-) is added

OH- CH3COOH (weak acid) in buffer solution reacts with the OH- ion added

CH3COOH CH3COOH (aq) + OH-(aq) → CH3COO- (aq) + H2O (l)
CH3COO-
Base (OH-) added is completely consumed. concentrations of
CH3COO- Concentration of [CH3COOH] decrease, but
CH3COOH [CH3COO-] increase.
As a result, pH of the solution is not much affected.

C. HOW BASIC BUFFER MAINTAIN IT’S pH?

Basic Buffer: NH3 / NH4Cl

➢ NH3 – weak base NH4+ (aq) + OH- (aq) NH3
Undergoes partial dissociation
Dissociation equation: NH3 (aq) + H2O (l) NH4+
NH4+
➢ NH4Cl – conjugate acid (salt)
Undergoes completely dissociation NH3
Dissociation equation: NH4Cl (aq) → NH4+ (aq) + Cl- (aq)

Case #1: When a small amount of strong acid (H+) is added

H+ NH3 (weak) in buffer solution reacts with the H+ ion added

NH3 NH3 (aq) + H+(aq) → NH4+ (aq)

NH4+ Acid (H+) added is completely consumed.
NH4+ Concentration of [NH4+] increase, but concentrations of [NH3]
decrease.
NH3 As a result, pH of the solution is not much affected.

Chemistry SK015 | KMK

7. Ionic Equilibria MADAM SUHAILA, CHEMISTRY 3

Case #2: When a small amount of strong base (OH-) is added

OH- NH4+ (conjugate acid) in buffer solution reacts with the OH- ion added

NH3 NH4+ (aq) + OH-(aq) → NH3 (aq) + H2O (l)

NH4+ Base (OH-) added is completely consumed.
NH4+ Concentration of [NH4+] decrease, but concentrations of [NH3]
increase.
NH3 As a result, pH of the solution is not much affected.

D. HANDERSON-HASSELBALCH EQUATION
- Calculation pH for buffer solution

➢ For acidic buffer solution

[conjugate base]
pH = pKa + log [weak acid]

➢ For basic buffer solution

pOH = pKb + log [conjugate acid]
[weak base]

 LET’S TRY

Example 1:
a) Calculate the pH of a solution containing 0.20 M CH3COOH and 0.3 M CH3COONa.
b) What would the pH of a 0.20 M of CH3COOH solution be if no salt were present?
(Ka CH3COOH = 1.8 x 10-5)

Example 2:
Calculate the pH of the solution prepared by mixing 500 mL 0.1 M hydrazinium chloride, N2H5Cl
with 500 mL 0.2 m hydrazine, N2H4. (Kb N2H4 = 1.7 x 10-7)

Example 3
A buffer solution is prepared by adding 200 mL 0.60 M NH3 solution to 300 mL 0.15 M NH4Cl
solution. Calculate:

a) the pH of this buffer solution
b) the new pH if 10 mL 0.05 M HNO3 solution is added to the original buffer solution.
(Kb = 1.8 x 10-5)

Example 4
What mass of CH3COONa must be added to 1.0 mol of CH3COOH to prepare a 1 dm3 buffer
solution with pH 5.0? (Ka = 1.8 x 10-5)

Chemistry SK015 | KMK

7. Ionic Equilibria MADAM SUHAILA, CHEMISTRY 4

Exercise (Buffer solution)

1 What is the pH of a buffer solution containing 0.245 M CH3NH2 and 0.186 M (10.8)

CH3NH3Cl?
(Kb CH3NH2 = 4.4 x 10-4)

2 A buffer solution with pH 4.50 is prepared by mixing propanoic acid and sodium

propanoate. If the concentration of sodium propanoate used is 0.50M, calculate the

concentration of propanoic acid.

(Ka CH3CH2COOH = 1.5 x 10-5) (1.05M)

3 Determine the ratio of [NH3] to [NH4+] for a buffer solution with pH 10.0. (5.56)
(Kb NH3 = 1.8 x 10-5)

4 What mass of (NH4)2SO4 must be dissolved in 0.100 L of 0.350 M NH3 to produce a

buffer solution with pH 10.05?

(Kb NH3 = 1.8 x 10-5) (0.37 g)

5 A 1.0 dm3 buffer solution consists of 0.45 mole of HCOOH and 0.55 mol of HCOONa.

Calculate pH of the buffer solution

a) change in pH if 0.10 mol of HCl is added

b) change in pH if 0.20 mol of NaOH is added

(Ka HCOOH = 1.1 x 10-4) (0.18, 0.38)

Chemistry SK015 | KMK