---
title: "Buffer criteria; sodium benzoate calculation"
subject: "Engineering Chemistry"
chapter: 1
key: "semester_2/engineering_chemistry:buffer-sod-benzoate"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/buffer-sod-benzoate
site: BCT Engineering
---

# Buffer criteria; sodium benzoate calculation

Engineering Chemistry · Chapter 1: Electro-chemistry and Buffer

- Asked in: 2074 Bhadra, 1+4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/buffer-sod-benzoate
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-1

## Question

What are the criteria for buffer system? How many grams of sodium benzoate should be added to 160 mL of 0.13 M benzoic acid solution to obtain a buffer with a pH of 4.3? [pKa value of benzoic acid is 4.2]

## Short answer

A buffer needs a weak acid-conjugate base pair, stable pH near pKa, and adequate capacity. Using pH = pKa + log([salt]/[acid]), 4.3 = 4.2 + log([salt]/0.13) yields [salt]=0.1637 M. For 160 mL, mass = 0.02619 mol × 144.1 g/mol = 3.77 g.

## Answer

#### Criteria for a buffer system
- It must resist a change in pH when a small amount of acid or alkali is added.
- It must contain a weak acid and its conjugate base (salt), or a weak base and its conjugate acid, in appreciable amounts.
- The ratio of the two components should be close to 1 (between 0.1 and 10), so that pH = pKa ± 1.
- The pH should not change much on dilution or on keeping for a long time.
- The components must not react with each other and must be stable; the buffer should have a reasonable buffer capacity.

#### Calculation
Benzoic acid (C₆H₅COOH) + sodium benzoate (C₆H₅COONa) form an acidic buffer.

```text
pH = pKa + log([salt]/[acid])
4.3 = 4.2 + log([salt]/[acid])
log([salt]/[acid]) = 0.1  →  [salt]/[acid] = 1.259
[salt] = 1.259 × 0.13 = 0.1637 M

Moles in 160 mL = 0.1637 × 160/1000 = 0.02619 mol
Molar mass of C₆H₅COONa = 144.1 g/mol
Mass = 0.02619 × 144.1 = 3.77 g
```
**Answer: about 3.77 g of sodium benzoate.** (Volume change on adding the solid is neglected.)

## Check yourself

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

1. For effective buffer action, the ratio of the two components should lie between:
   A. 0.01 and 100
   B. 0.1 and 10, so pH = pKa ± 1
   C. 1 and 100
   D. Exactly 1 only
   Answer: B. A ratio of 0.1 to 10 keeps pH within pKa ± 1 where the buffer is effective. (Key point: Buffer range criterion)

2. Which pair makes a valid buffer system?
   A. Strong acid and its salt
   B. Weak acid and its conjugate base (salt)
   C. Strong acid and strong base
   D. Weak acid and a strong acid
   Answer: B. A buffer needs a weak acid and its conjugate base (or a weak base and its conjugate acid) in appreciable amounts. (Key point: Components of a buffer)

3. For pH 4.3 with pKa 4.2, what is [salt]/[acid]?
   A. 0.794
   B. 1.259
   C. 1.0
   D. 10
   Answer: B. log ratio = 4.3 - 4.2 = 0.1, so ratio = 10^0.1 = 1.259. (Key point: Required salt/acid ratio)

4. [salt] = 0.1637 M in 160 mL, molar mass 144.1 g/mol. What mass of sodium benzoate is needed?
   A. 23.6 g
   B. 3.77 g
   C. 0.026 g
   D. 37.7 g
   Answer: B. Moles = 0.1637 x 0.160 = 0.02619 mol; mass = 0.02619 x 144.1 = 3.77 g. (Key point: Converting to mass)

## Related in Engineering Chemistry

- [Buffer capacity; benzoic acid-benzoate buffer](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Abuffer-capacity-benzoate) — Engineering Chemistry, chapter 1
- [Buffer capacity and range; sodium benzoate concentration](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Abuffer-capacity-range) — Engineering Chemistry, chapter 1
- [Buffer solution; sodium formate for pH 3.80](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Abuffer-sodium-formate) — Engineering Chemistry, chapter 1
- [Buffer action; pH after adding KOH to acetic acid buffer](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aacidic-buffer-koh-ph) — Engineering Chemistry, chapter 1
- [Buffer; pH and degree of ionization of acetic acid](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aacetate-buffer-ionization) — Engineering Chemistry, chapter 1
- [Henderson equation; pH after NaOH added to acetic acid](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ahenderson-basic-naoh-acetic) — Engineering Chemistry, chapter 1