---
title: "pH of mixture: 700 ml pH 6 and 500 ml pH 8"
subject: "Water Supply Engineering"
chapter: 4
key: "semester_5/water_supply_engineering:ph-mix-c76"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/ph-mix-c76
site: BCT Engineering
---

# pH of mixture: 700 ml pH 6 and 500 ml pH 8

Water Supply Engineering · Chapter 4: Quality of Water

- Asked in: 2076 Chaitra, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/ph-mix-c76
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-4

## Question

A water sample of 700 ml with pH 6 is mixed to another water sample of 500 ml with pH 8. What will be the pH of the mixture?

## Short answer

Combine hydrogen-ion amounts: final H+ = (700e-6 + 500e-8)/1200 = 5.875e-7 mol/l. Taking the negative log gives pH = 6.23.

## Answer

pH is $-\log_{10}[H^+]$. When two samples are mixed, the hydrogen-ion amounts (moles) add, so the final $[H^+]$ is the volume-weighted average. (Buffering by alkalinity is ignored, as is usual in this type of problem.)

Sample 1: 700 ml, pH 6, so $[H^+] = 10^{-6}$ mol/l.
Sample 2: 500 ml, pH 8, so $[H^+] = 10^{-8}$ mol/l.

$$
\begin{aligned} [H^+]_{mix} &= \frac{700\times10^{-6} + 500\times10^{-8}}{700+500} \\ &= \frac{7.0\times10^{-4} + 5.0\times10^{-6}}{1200} = 5.875\times10^{-7}\ \text{mol/l} \end{aligned}
$$

$$
pH = -\log_{10}(5.875\times10^{-7}) = 6.23
$$

**Answer: pH of the mixture = 6.23** (acidic, because the acidic sample dominates).

## Check yourself

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

1. On mixing two samples, what is averaged?
   A. [H+], weighted by volume
   B. pH, simple average
   C. [OH-] only
   D. pH, weighted by volume
   Answer: A. Moles of H+ add, so [H+] (not pH) is the volume-weighted average. (Key point: Mixing rule)

2. What is [H+] for the 500 ml sample at pH 8?
   A. 10^8 mol/l
   B. 10^-8 mol/l
   C. 0.8 mol/l
   D. 8 mol/l
   Answer: B. [H+] = 10^-pH. (Key point: Concentration from pH)

3. Why does the mixture (pH 6.23) come out acidic, near pH 6?
   A. The acidic sample has far more H+ so it dominates
   B. pH values simply average to 7
   C. Base neutralises all acid
   D. Larger volume always sets the pH
   Answer: A. The pH 6 sample has 100 times more H+ per litre, so it dominates the weighted average. (Key point: Result)

## Related in Water Supply Engineering

- [pH of mixture: 400 ml pH 6 and 700 ml pH 8](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aph-mix-c69) — Water Supply Engineering, chapter 4
- [pH of mixture of three samples](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aph-mix-b66) — Water Supply Engineering, chapter 4

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