---
title: "Buffer capacity; benzoic acid-benzoate buffer"
subject: "Engineering Chemistry"
chapter: 1
key: "semester_2/engineering_chemistry:buffer-capacity-benzoate"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/buffer-capacity-benzoate
site: BCT Engineering
---

# Buffer capacity; benzoic acid-benzoate buffer

Engineering Chemistry · Chapter 1: Electro-chemistry and Buffer

- Asked in: 2072 Asoj, 3 marks
- Video: [Henderson-Hasselbalch Equation Explained | pH & Buffer Example Problem](https://www.youtube.com/watch?v=Y3htAlEOFv0) — Chemistry TowleVision
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/buffer-capacity-benzoate
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-1

## Question

What is meant by buffer capacity of a buffer solution? How does a solution containing a mixture of benzoic acid and sodium benzoate maintain its constant pH value even on the addition of small amount of strong acid or alkali? Explain.

## Short answer

Buffer capacity is the moles of strong acid or base needed to change 1 L of buffer pH by one unit. In benzoic acid/benzoate buffer, added H⁺ is consumed by C₆H₅COO⁻ to form C₆H₅COOH, and added OH⁻ reacts with C₆H₅COOH to give C₆H₅COO⁻ and water; the salt/acid ratio stays nearly constant, preserving pH.

## Answer

#### Buffer capacity
The **buffer capacity** (buffer index) is the number of moles of strong acid or strong base needed to change the pH of one litre of the buffer by one unit. A buffer has maximum capacity when [salt] = [acid] (pH = pKa) and when the concentrations of its components are high.

#### Action of benzoic acid / sodium benzoate buffer
The solution contains the weak acid C₆H₅COOH (mostly undissociated) and a large concentration of benzoate ions, C₆H₅COO⁻, from the fully ionised salt C₆H₅COONa.

- **On adding a small amount of strong acid (H⁺):** the benzoate ions combine with H⁺ to give unionised benzoic acid, so [H⁺] hardly increases.
  C₆H₅COO⁻ + H⁺ → C₆H₅COOH
- **On adding a small amount of strong alkali (OH⁻):** OH⁻ reacts with the benzoic acid to give benzoate and water, so [OH⁻] hardly increases.
  C₆H₅COOH + OH⁻ → C₆H₅COO⁻ + H₂O

In both cases the ratio [C₆H₅COO⁻]/[C₆H₅COOH] changes only a little, and pH = pKa + log([salt]/[acid]) remains almost constant.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Buffer capacity is:
   A. The pH of the buffer
   B. Moles of strong acid or base needed to change pH of 1 L of buffer by one unit
   C. The volume of buffer that can be prepared
   D. The ratio of Ka to Kb
   Answer: B. Buffer capacity measures how much acid or base the buffer absorbs for a 1 unit pH change in 1 L. (Key point: Buffer capacity definition)

2. A buffer has maximum capacity when:
   A. [salt] much greater than [acid]
   B. [salt] = [acid] and concentrations are high
   C. [acid] is zero
   D. pH = 14
   Answer: B. Capacity is highest at pH = pKa (equal components) with large concentrations. (Key point: Maximum capacity)

3. In the benzoic acid/benzoate buffer, what happens to added H+?
   A. Benzoic acid ionises more
   B. Benzoate ions combine with H+ to form benzoic acid
   C. Sodium ions neutralise it
   D. It forms H2
   Answer: B. C6H5COO- + H+ -> C6H5COOH removes H+, so [H+] hardly rises. (Key point: Response to added H+)

4. What reacts with added OH- in this buffer?
   A. Benzoate ions
   B. Benzoic acid, giving benzoate and water
   C. Sodium ions
   D. Water only
   Answer: B. C6H5COOH + OH- -> C6H5COO- + H2O removes OH-. (Key point: Response to added OH-)

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