The pH of 10⁻⁸M HNO₃ solution in water is 

Analysis of the Problem

The pH of a solution is determined by the concentration of hydrogen ions (H⁺) in the solution. In the case of a very dilute acid like 10⁻⁸ M HNO₃, we must also account for the H⁺ ions contributed by the dissociation of water, as the contribution from the acid is comparable to the ionization of water.

Steps to Solve

1. Contribution from HNO₃: HNO₃ is a strong acid and dissociates completely, contributing 10⁻⁸ M of H⁺ ions to the solution.
2. Contribution from Water: Water autoionizes to give 10⁻⁷ M H⁺ ions at 25°C. Thus, the total H⁺ concentration is the sum of H⁺ from HNO₃ and water.
3. Total H⁺ Concentration:Total [H⁺] = [H⁺ from HNO₃] + [H⁺ from water] Total [H⁺] = 10⁻⁸ + 10⁻⁷ Total [H⁺] = 1.1 × 10⁻⁷ M
4. Calculate pH:pH is calculated as: pH = -log[H⁺] pH = -log(1.1 × 10⁻⁷) pH ≈ 6.95

Correct Answer

The correct answer is:

c) 6.95

Solution Recap

1. The H⁺ ions from HNO₃ contribute 10⁻⁸ M.
2. The H⁺ ions from water contribute 10⁻⁷ M.
3. Total [H⁺] = 1.1 × 10⁻⁷ M.
4. pH = -log(1.1 × 10⁻⁷) ≈ 6.95.

This confirms that the pH of the solution is approximately 6.95, making c) 6.95 the correct answer.