Question

Sometimes chlorine gas is passed through water for its purification. What will be the pH value of such a sample of water?

• A. Equal to 7
• B. More than 7
• C. Less than 7
• D. Very near to 8

Answer 1

The correct option is C

Less than 7

Explanation of correct answer

Water $\left({\mathrm{H}}_2\mathrm{O}\right)$ is a neutral liquid with a pH equal to 7. When Chlorine gas $\left({\mathrm{Cl}}_2\right)$ is passed through water, it reacts with water to form Hydrochloric acid $\left(\mathrm{HCl}\right)$ and Hypochlorous acid $\left(\mathrm{HOCl}\right)$. The balanced equation for the reaction is:

$$\begin{gathered} {\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)\to \mathrm{HCl}\left(\mathrm{aq}\right)+\mathrm{HOCl}\left(\mathrm{aq}\right) \\ \left(\mathrm{Water}\right)\left(\mathrm{Chlorine}\right)(\mathrm{Hydrochloric}(\mathrm{Hypochlorous} \\ \mathrm{acid})\mathrm{acid}) \end{gathered}$$

The pH of Hydrochloric acid and Hypochlorous acid is less than 7. The presence of these acids decreases the pH of the water, and therefore, the sample of the water will have a pH of less than 7.

Explanations of incorrect answers

Since the presence of acids lowers the pH of water, the pH of the water sample cannot be 7 or greater than 7. Therefore, options (A), (B) and (D) are incorrect.

Therefore, the correct option is (C) less than 7.