---
title: "Buffer solution; pH of NH₄OH-NH₄Cl mixture"
subject: "Engineering Chemistry"
chapter: 1
key: "semester_2/engineering_chemistry:buffer-ph-nh4oh-nh4cl"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/buffer-ph-nh4oh-nh4cl
site: BCT Engineering
---

# Buffer solution; pH of NH₄OH-NH₄Cl mixture

Engineering Chemistry · Chapter 1: Electro-chemistry and Buffer

- Asked in: 2070 Magh, 1+4 marks
- Video: [Mechanism of a Basic Buffer (Buffer Solutions-5)](https://www.youtube.com/watch?v=l8iz_SUgzbI) — JSM Learn
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/buffer-ph-nh4oh-nh4cl
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-1

## Question

What is a buffer solution? Calculate the pH of the resulting buffer by mixing 400 ml of 0.2 M ammonium hydroxide and 150 ml of 0.3 M ammonium chloride solution. [Kb = 1.8×10⁻⁵]

## Short answer

A buffer resists pH change upon acid/base addition. Moles NH4OH=0.080, NH4Cl=0.045, pKb=4.745, pOH=4.745+log(0.045/0.080)=4.495, so pH=14−4.495=9.51.

## Answer

A **buffer solution** is a solution that resists a change in its pH when a small amount of a strong acid or strong base is added, or when it is diluted. It contains a weak acid with its salt (acidic buffer) or a weak base with its salt (basic buffer).

#### Calculation
NH₄OH (weak base) and NH₄Cl (its salt) form a basic buffer.
```text
Moles NH₄OH = 0.2 × 400/1000 = 0.080 mol
Moles NH₄Cl = 0.3 × 150/1000 = 0.045 mol
(Both are in the same final volume, so the ratio of moles = ratio of concentrations.)

pKb = −log(1.8×10⁻⁵) = 4.745
pOH = pKb + log([salt]/[base])
    = 4.745 + log(0.045/0.080)
    = 4.745 − 0.250 = 4.495
pH  = 14 − 4.495 = 9.505
```
**Answer: pH ≈ 9.51**

## Check yourself

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

1. A buffer solution:
   A. Resists change in pH on adding small amount of acid or base
   B. Keeps pH at 7
   C. Has pH that changes strongly on dilution
   D. Contains only a strong acid
   Answer: A. It resists pH change on small additions of strong acid/base or dilution. (Key point: Definition)

2. NH4OH + NH4Cl is a:
   A. Basic buffer
   B. Acidic buffer
   C. Neutral buffer
   D. Not a buffer
   Answer: A. A weak base with its salt gives a basic buffer. (Key point: Type of buffer)

3. Moles of NH4OH and NH4Cl are:
   A. 0.080 and 0.045
   B. 0.2 and 0.3
   C. 0.045 and 0.080
   D. 0.80 and 0.45
   Answer: A. 0.2 x 0.4 = 0.080 mol; 0.3 x 0.15 = 0.045 mol. (Key point: Moles)

4. pKb = 4.745, ratio salt/base = 0.045/0.080. pH =
   A. 9.51
   B. 4.50
   C. 14.0
   D. 8.99
   Answer: A. pOH = 4.745 - 0.250 = 4.495; pH = 14 - 4.495 = 9.505. (Key point: pH)

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