Chapter 17: Acid-Base Equilibria
Chapter 17: Acid-Base Equilibria

Buffer
One of the fundamental concepts of acid-base is the buffer reactions.
A buffer is a solution which pH remains almost unchanged when a small amount of H⁺ or OH⁻ is added to it. This definition is extremely important and should be memorised. There are 2 types of buffers. The table below will provide more explanation of each type of buffer.
Acidic buffer | Basic buffer |
---|---|
A solution of a weak acid and its conjugate base Example: CH₃COOH (weak acid) and CH₃COONa (contains the conjugate base CH₃COO⁻) (I) On adding a small amount of H⁺, (II) On adding a small amount of OH⁻, |
A solution of a weak base and its conjugate acid Example: NH₃ (weak base) and NH₄Cl (contains the conjugate acid NH₄⁺) (I) On adding a small amount of OH⁻ (II) On adding a small amount of H⁺ |
In order to calculate the pH of the buffer, we should utilise the Henderson-Hasselbalch Equation (HHE for short). | |
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Acidic buffer |
---|
A solution of a weak acid and its conjugate base Example: CH₃COOH (weak acid) and CH₃COONa (contains the conjugate base CH₃COO⁻) (I) On adding a small amount of H⁺, (II) On adding a small amount of OH⁻, |
Basic buffer |
A solution of a weak base and its conjugate acid Example: NH₃ (weak base) and NH₄Cl (contains the conjugate acid NH₄⁺) (I) On adding a small amount of OH⁻ (II) On adding a small amount of H⁺ |
In order to calculate the pH of the buffer, we should utilise the Henderson-Hasselbalch Equation (HHE for short). |
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Steps in tackling calculation questions regarding buffer solutions
❏ Identify the type of buffer (whether acidic or basic buffer)
❏ Determine [acid] or [base]
❏ Determine [salt] in the buffer
❏ Calculate pH using the appropriate Henderson-Hasselbalch equation
Let's take a look at worked example 1 to illustrate pH calculations in a buffer solution:
Worked Example 1
Calculate the pH of a 1 dm³ solution prepared by mixing 0.04 mol of NaOH and 0.10 mol of CH₃COOH. The pKₐ of CH₃COOH is 4.74.
Solution:
Upon mixing, an acid base reaction occurs following the equation
OH⁻ (aq) + CH₃COOH (aq) → CH₃COO⁻ (aq) + H₂O (l)
Since initial n(CH₃COOH) > n(OH⁻) added, some CH₃COOH remains unreacted and the resultant mixture contains both CH₃COOH and CH₃COO⁻ , hence an acidic buffer is formed.
After mixing,
n(CH₃COOH) left = initial n(CH₃COOH) - n(CH₃COOH) reacted
= 0.10 - 0.04
= 0.06 mol
n(CH₃COO⁻) = n(OH⁻) added = 0.04 mol
Hence [CH₃COOH] = 0.06 mol dm⁻³ and [CH₃COO⁻] = 0.04 mol dm⁻³
For this acidic buffer,
pH = pKₐ + lg([CH₃COO⁻]/[CH₃COOH])
- = 4.74 + lg(0.04/0.06)
- = 4.56
Acid-Base Titration Curves
The acid-base titration curves help us to explain and understand the changes in pH during
acid-base titrations better. Students are required to know how to draw the shapes of the titration
graphs based on the given scenarios in the question and/or practical paper 4.
Do note that only 4 possible types of titration curves can be derived from any acid -base titration experiment:
- 1. Strong acid titrated against strong base
- 2. Weak acid titrated against strong base
- 3. Strong acid titrated against weak base
- 4. Weak acid titrated against weak base
Checklist to ensure a perfect titration curve drawing:
❏ Labelling of axes (pH and Volume of titrant added/cm³)
❏ Indicate the equivalence point(s), and label the pH and volume of titrant added at that point
❏ Indicate the maximum buffer capacity and label volume of titrant added and pH of solution at that point
❏ Label the initial pH
❏ Label the pH of solution when the excess volume of acid or base is added (depends if the question provided the information)
The following is a summary of the Acid-Base Titration graphs:
Strong Acid-Strong Base |
Strong base titrated against strong acid![]() |
Strong acid titrated against strong base![]() |
---|---|---|
Weak Acid-Strong Base |
Weak base titrated against strong
acid
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Strong base titrated against weak acid![]() |
Strong Acid-Weak Base |
Strong base titrated against weak acid
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Weak acid titrated against strong
base
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Strong Acid-Strong Base |
---|
Strong base titrated against strong acid![]() |
Strong acid titrated against strong base![]() |
Weak Acid-Strong Base |
---|
Weak base titrated against strong
acid
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Strong acid titrated against weak
base
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Strong Acid-Weak Base |
---|
Strong base titrated against weak
acid
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Weak acid titrated against strong
base
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We will explain to you in-depth using a sample worked example below to understand it better through application:
Worked Example 2
[2019 RVHS JC2 H2 Chemistry Paper 3 Qn2(b)(i)(ii)(iii)]

Phthalic anhydride is the acid anhydride of phthalic acid. It is an important industrial chemical for the synthesis of phthalic esters, which are used as plasticisers to soften plastics.
Hydrolysis of phthalic anhydride produces phthalic acid, which is an aromatic dicarboxylic acid that can ionise in stages. Some information on phthalic acid and another aromatic acid are given below.



Table 2.1
pK₁ | pK₂ | |
---|---|---|
Phthalic acid | 2.89 | 5.51 |
Benzoic acid | 4.20 | - |
(i) Suggest a reason why the pK₂ of phthalic acid is higher than its pK₁.
(ii) Calculate the pH of a solution that is obtained after 50 cm³ of 0.1 mol dm⁻³ of NaOH is added to 10 cm³ of 0.2 mol dm⁻³ phthalic acid.
(iii) Sketch the pH-volume added curve you would expect to obtain when 50 cm³ of 0.1 mol dm⁻³ of NaOH is added to 10 cm³ of 0.2 mol dm⁻³ phthalic acid. Include relevant details from (ii) on your graph.
Solution:
(i) For pK₁, the H⁺ is removed from a neutral molecule (referring to phthalic acid). For pK₂, the removal of a H⁺ from the anion that already carries a negative charge is electrostatically unfavourable.
OR
Favourable intramolecular hydrogen bonding in the anion will be disrupted when the second H⁺ dissociates in pK₂. (Refer to the diagram below for a better understanding of the second alternative answer.)

(ii) Phthalic acid is the limiting reagent / NaOH is in excess

pOH = lg(1.667 × 10⁻² ) = 1.78
pH = 14 - 1.78 = 12.2
(iii) Step-By-Step Guide:
Firstly, you need to recall what pKₐ is. pKₐ is a way to express the extent of dissociation. The lower the pKₐ, the stronger the acid.
Phthalic acid has 2 pKₐ values, meaning that this is a polyprotic acid-base titration and phthalic acid underwent two dissociations with 2 acidic protons.
In this case, the value of pK₁ of phthalic acid is smaller than the value of pK₂ of phthalic acid, so we can infer that the solution is getting more basic as we proceed on with the dissociations, and the titration curve will move upwards (increasing pH).
Next, the question mentioned “50 cm³ of 0.1 mol dm⁻³ of NaOH is added to 10 cm³ of 0.2 mol dm⁻³ phthalic acid”, so it is a weak acid-strong base titration. Recall the titration curve shape.
Do remember to label the pH values that are known to you already.
Initial pH = pH of phthalic acid = unknown
1ˢᵗ buffer pH = pK₁ = 2.89 (given)
2ⁿᵈ buffer pH = pK₂ = 5.51 (given)
pH of solution when 50 cm³ of NaOH is added: 12.2 [calculated in (ii)]
Maximum Buffer Capacity (half-equivalence point):
The horizontal parts of the curve are where Maximum Buffer Capacity (MBC) occurs, which is when [weak acid] = [conjugate base]. In this case, the buffers formed are acidic buffers, thus pH= pKₐ. You need to know that this point occurs at half the volume of base required to neutralise the acid completely.
At every equivalence point, all the weak acid in the conical flask would be completely neutralised. However, strong base NaOH will still be added, so what’s left in the flask is the salt formed and excess strong NaOH base.
Therefore, the titration curve should be as such:

Lastly, here is a summary table of the commonly used indicators in acid-base. This table would also be useful in tackling the practical portion of H2 Chemistry.
Indicator | Colour |
pH range for colour change/working range |
|
---|---|---|---|
Acid | Base | ||
Methyl orange | Red | Yellow | 3.2 - 4.4 |
Screened methyl orange | Violet | Green | 3.2 - 4.4 |
Phenolphthalein | Colourless | Pink | 8.2 - 10.0 |
Thymolphthalein | Colourless | Blue | 9.3 - 10.5 |
Indicator | Methyl orange | |
---|---|---|
Colour | Acid | Red |
Base | Yellow | |
pH range for colour change/working range | 3.2 - 4.4 |
Indicator | Screened methyl orange | |
---|---|---|
Colour | Acid | Violet |
Base | Green | |
pH range for colour change/working range | 3.2 - 4.4 |
Indicator | Phenolphthalein | |
---|---|---|
Colour | Acid | Colourless |
Base | Pink | |
pH range for colour change/working range | 8.2 - 10.0 |
Indicator | Thymolphthalein | |
---|---|---|
Colour | Acid | Colourless |
Base | Blue | |
pH range for colour change/working range | 9.3 - 10.5 |