---
title: "Buffer action; pH after adding KOH to acetic acid buffer"
subject: "Engineering Chemistry"
chapter: 1
key: "semester_2/engineering_chemistry:acidic-buffer-koh-ph"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/acidic-buffer-koh-ph
site: BCT Engineering
---

# Buffer action; pH after adding KOH to acetic acid buffer

Engineering Chemistry · Chapter 1: Electro-chemistry and Buffer

- Asked in: 2071 Magh, 1+2+2 marks
- Video: [Buffer solution pH calculations | Chemistry | Khan Academy](https://www.youtube.com/watch?v=lsHq5aqz4uQ) — Khan Academy Organic Chemistry
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/acidic-buffer-koh-ph
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-1

## Question

What is buffer solution? How acidic buffer resists the change in pH on the addition of small amount of acid and base? 0.005 mole of KOH is added to 400 ml of 0.1 M CH₃COOH. Calculate the pH of the mixture. Given pKa = 4.74.

## Short answer

A buffer solution resists pH change. An acidic buffer neutralizes added H+ with CH3COO- and OH- with CH3COOH. Initial CH3COOH 0.040 mol, after 0.005 mol KOH, acid 0.035 mol, salt 0.005 mol. pH = 4.74 + log(0.005/0.035) = 3.90.

## Answer

#### Buffer solution
A solution which resists a change in pH on adding small amounts of acid or base. An acidic buffer is a weak acid with its salt, e.g. CH₃COOH + CH₃COONa.

#### Action of acidic buffer
The solution has much CH₃COOH (acid reserve) and CH₃COO⁻ from the salt (base reserve).
- **Small amount of acid (H⁺) added:** H⁺ is taken up by acetate ions: CH₃COO⁻ + H⁺ → CH₃COOH. The weak acid is formed, so pH hardly falls.
- **Small amount of base (OH⁻) added:** OH⁻ is neutralised by acetic acid: CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O. So pH hardly rises.

The ratio [salt]/[acid] changes little, so pH = pKa + log([salt]/[acid]) is almost constant.

#### Calculation
```text
Moles CH₃COOH = 0.1 × 0.400 = 0.040 mol
KOH added = 0.005 mol
CH₃COOH + KOH → CH₃COOK + H₂O
Acid left = 0.040 − 0.005 = 0.035 mol
Salt formed = 0.005 mol

pH = 4.74 + log(0.005/0.035)
   = 4.74 − 0.845 = 3.895
```
**Answer: pH ≈ 3.90**

## Check yourself

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

1. When a small amount of acid is added to the acetic acid/acetate buffer, the H+ is removed by:
   A. CH3COOH forming CH3COO-
   B. Water
   C. K+
   D. CH3COO- forming CH3COOH
   Answer: D. The acetate (base reserve) converts H+ into weak acetic acid, so pH hardly falls. (Key point: Acid addition to buffer)

2. When OH- is added, it is neutralised by:
   A. Acetate ions
   B. Water
   C. Sodium ions
   D. Acetic acid, forming acetate and water
   Answer: D. CH3COOH + OH- -> CH3COO- + H2O, so pH hardly rises. (Key point: Base addition to buffer)

3. 0.005 mol KOH added to 0.040 mol CH3COOH. Acid and salt present:
   A. Acid 0.040 mol, salt 0.005 mol
   B. Acid 0.005 mol, salt 0.035 mol
   C. Acid 0.045 mol, salt 0.005 mol
   D. Acid 0.035 mol, salt 0.005 mol
   Answer: D. KOH consumes an equal amount of acid and forms salt. (Key point: Amounts after KOH)

4. pKa 4.74, salt 0.005, acid 0.035. pH =
   A. 5.59
   B. 4.74
   C. 4.30
   D. 3.90
   Answer: D. pH = 4.74 + log(1/7) = 4.74 - 0.845 = 3.895, about 3.90. (Key point: Final pH)

## Related in Engineering Chemistry

- [Buffer action; pH change on adding KOH](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Abuffer-koh-change-ph) — Engineering Chemistry, chapter 1
- [Henderson equation; pH after adding NaOH to acetic acid](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ahenderson-acidic-ph) — 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
- [Corrosion; pH after adding NaOH to acetic acid](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Acorrosion-ph-naoh-acetic) — 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
- [pH of acetic acid and NaOH mixture](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aph-acetic-naoh) — Engineering Chemistry, chapter 1

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