Question

pH of 10⁻⁸ M HCl solution is:

• A. 8
• B. Greater than 7, less than 8
• C. Greater than 8
• D. Greater than 6, less than 7

Hint:

For very dilute strong acids, account for the autoionization of water when calculating pH.

Answer 1

The Correct Option is D

The concentration of HCl is \(10^{-8}\) M. Since HCl is a strong acid, the [H⁺] from HCl is \(10^{-8}\) M. However, the autoionization of water also contributes [H⁺] = \(10^{-7}\) M. Therefore, the total [H⁺] is:

\( \text{Total} [\text{H}^+] = 10^{-8} + 10^{-7} \approx 1.1 \times 10^{-7} \text{ M} \)

The pH is given by:

\( \text{pH} = -\log[\text{H}^+] \approx -\log(1.1 \times 10^{-7}) \approx 6.96 \)

Thus, the pH is greater than 6 and less than 7.

Answer 2

pH of 10^-8 M HCl solution is:

• Option 1: 8
• Option 2: Greater than 7, less than 8
• Option 3: Greater than 8
• Option 4: Greater than 6, less than 7

Correct Answer:

Option 4: Greater than 6, less than 7

Explanation:

Even though the HCl concentration is 10^-8 M, we must consider the autoionization of water, which contributes 10^-7 M H⁺ ions.

The total [H⁺] is approximately 1.1 x 10^-7 M.

pH = -log(1.1 x 10^-7) ≈ 6.96.

Therefore, the pH is greater than 6 but less than 7.