---
title: "Buffer criteria; NH₄Cl weight calculation"
subject: "Engineering Chemistry"
chapter: 1
key: "semester_2/engineering_chemistry:buffer-nh4cl"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/buffer-nh4cl
site: BCT Engineering
---

# Buffer criteria; NH₄Cl weight calculation

Engineering Chemistry · Chapter 1: Electro-chemistry and Buffer

- Asked in: 2073 Bhadra, 2+3 marks
- Video: [How To Prepare Buffer Solution||Ammonium chloride - Ammonium hydroxide Buffer Solution...](https://www.youtube.com/watch?v=HbwIBDu15Tw) —  Ruchi Upadhyay Classes 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/buffer-nh4cl
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-1

## Question

What are the criteria for buffer system? Calculate the weight in gram of NH₄Cl required to prepare buffer solution with 2 litre of 0.2N NH₄OH solution with pH = 9. (Kb = 1.8×10⁻⁵)

## Short answer

A buffer requires a weak base-conjugate acid pair, stable pOH, and capacity. For NH₄OH buffer pH 9, pOH = 5, [OH⁻] = 10⁻⁵. Using [OH⁻] = Kb × [base]/[salt] gives [NH₄Cl] = (1.8×10⁻⁵ × 0.2) / 10⁻⁵ = 0.36 M. In 2 L, mass = 0.72 mol × 53.5 g/mol = 38.5 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 (basic buffer NH₄OH + NH₄Cl)
Given: NH₄OH = 0.2 N (0.2 M, as NH₄OH has one OH⁻), volume = 2 L, pH = 9, Kb = 1.8×10⁻⁵.

```text
pOH = 14 − 9 = 5   →  [OH⁻] = 10⁻⁵ M
For a basic buffer: [OH⁻] = Kb × [base]/[salt]
10⁻⁵ = 1.8×10⁻⁵ × 0.2/[NH₄Cl]
[NH₄Cl] = 1.8×10⁻⁵ × 0.2 / 10⁻⁵ = 0.36 M

Moles of NH₄Cl in 2 L = 0.36 × 2 = 0.72 mol
Molar mass NH₄Cl = 53.5 g/mol
Mass = 0.72 × 53.5 = 38.52 g
```
**Answer: about 38.5 g of NH₄Cl.**

## Check yourself

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

1. Which is NOT a criterion of a good buffer?
   A. Contains weak base and its conjugate acid in appreciable amounts
   B. pH changes greatly on dilution
   C. Components do not react with each other
   D. Ratio of components close to 1 (0.1 to 10)
   Answer: B. A good buffer must keep pH nearly unchanged on dilution. (Key point: Buffer criteria)

2. For a NH4OH/NH4Cl buffer, which expression is used?
   A. [OH-] = Kb [salt]/[base]
   B. [OH-] = Kb [base]/[salt]
   C. [H+] = Kb [base]/[salt]
   D. [OH-] = Kb x [base] x [salt]
   Answer: B. From Kb = [NH4+][OH-]/[NH3], [OH-] = Kb [base]/[salt]. (Key point: Basic buffer formula)

3. For pH = 9, what is [OH-]?
   A. 10^-9 M
   B. 10^-5 M
   C. 10^-14 M
   D. 10^-7 M
   Answer: B. pOH = 14 - 9 = 5, so [OH-] = 10^-5 M. (Key point: Converting pH to [OH-])

4. [NH4Cl] = 0.36 M in 2 L, molar mass 53.5 g/mol. What mass is required?
   A. 19.3 g
   B. 38.5 g
   C. 9.6 g
   D. 77 g
   Answer: B. Moles = 0.36 x 2 = 0.72 mol; mass = 0.72 x 53.5 = 38.5 g. (Key point: Mass of NH4Cl)

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