Question

pH of a neutral solution at 25°C is: ____.

• A. 0
• B. 5
• C. 7
• D. 14

Hint:

The "neutral" point of 7 is temperature-dependent. If the temperature increases, the $K_w$ of water changes, and the neutral pH value actually drops below 7, even though the water remains neutral!

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The pH scale measures the acidity or alkalinity of a solution based on the concentration of hydrogen ions $[H^+]$. The scale typically ranges from 0 to 14.

Step 2: Detailed Explanation:
At a standard temperature of 25°C ($298\text{ K}$), the ionic product of water ($K_w$) is $1.0 \times 10^{-14}$.
• For a neutral solution: $[H^+] = [OH^-]$.
• Therefore, $[H^+]^2 = 1.0 \times 10^{-14}$, which means $[H^+] = 10^{-7}\text{ mol/L}$.
• $pH = -\log[H^+] = -\log(10^{-7}) = 7$.

Step 3: Final Answer:
The pH of a neutral solution at 25°C is 7.