---
title: "Buffer solution; sodium formate for pH 3.80"
subject: "Engineering Chemistry"
chapter: 1
key: "semester_2/engineering_chemistry:buffer-sodium-formate"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/buffer-sodium-formate
site: BCT Engineering
---

# Buffer solution; sodium formate for pH 3.80

Engineering Chemistry · Chapter 1: Electro-chemistry and Buffer

- Asked in: 2069 Bhadra, 1+4 marks
- Video: [, To 50  mL of 0.05 M formic acid, how much volume of 0.10 M sodium formate must be added to geta...](https://www.youtube.com/watch?v=nFgzQTYv8I0) — PW Solutions
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/buffer-sodium-formate
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-1

## Question

What is meant by buffer solution? Calculate the concentration of sodium formate, HCOONa, that must be present in a 0.10M solution of formic acid to produce a pH of 3.80. [Ka for formic acid is 1.8×10⁻⁴].

## Short answer

A buffer resists pH change, containing a weak acid and its salt. Using [H⁺] = Ka[acid]/[salt] with [H⁺] = 10⁻³·⁸ M, the concentration of sodium formate needed is (1.8×10⁻⁴ × 0.10) / 1.58×10⁻⁴ ≈ 0.114 M.

## Answer

#### Buffer solution
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
Formic acid HCOOH + sodium formate HCOONa form an acidic buffer.
```text
pH = 3.80  →  [H⁺] = 10⁻³·⁸ = 1.585×10⁻⁴ M
[H⁺] = Ka [acid]/[salt]
[salt] = Ka [acid]/[H⁺]
       = 1.8×10⁻⁴ × 0.10 / 1.585×10⁻⁴
       = 0.1136 M
```
Check: pKa = 3.745; log([salt]/[acid]) = 3.80 − 3.745 = 0.055; ratio = 1.136; [salt] = 0.1136 M.

**Answer: [HCOONa] ≈ 0.114 M.**

## Check yourself

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

1. A buffer resists change in pH on adding:
   A. Small amounts of strong acid or base, or on dilution
   B. Large excess of acid
   C. Only water
   D. Only salts
   Answer: A. That is the defining property of a buffer. (Key point: Definition)

2. HCOOH + HCOONa is:
   A. Acidic buffer
   B. Basic buffer
   C. Neutral solution
   D. Strong acid
   Answer: A. Weak acid with its salt. (Key point: Type)

3. pH 3.80 gives [H+] =
   A. 1.585x10^-4 M
   B. 3.8x10^-4 M
   C. 1.585x10^-3 M
   D. 6.3x10^-5 M
   Answer: A. 10^-3.8 = 1.585x10^-4 M. (Key point: [H+] from pH)

4. [salt] = Ka [acid]/[H+] with Ka 1.8x10^-4, acid 0.10 M:
   A. 0.114 M
   B. 0.088 M
   C. 1.14 M
   D. 0.018 M
   Answer: A. 1.8x10^-5/1.585x10^-4 = 0.1136 M. (Key point: Salt concentration)

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