Concentration of H+ ion is 10-9. What is the pH of the solution?

**Core Concept**
The pH of a solution is a measure of its acidity or alkalinity, calculated using the concentration of hydrogen ions (H+) in moles per liter (molarity). The pH is defined as the negative logarithm of the H+ ion concentration.

**Why the Correct Answer is Right**
Using the formula pH = -log[H+], we can calculate the pH of the solution. Given the concentration of H+ ions is 10^-9, we substitute this value into the formula: pH = -log(10^-9). Since the logarithm of 10^-9 is -9, the pH of the solution is 9. This means the solution is basic, as a pH above 7 indicates alkalinity.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided, so we cannot evaluate it.
* **Option B:** This option is also not provided, so we cannot evaluate it.
* **Option C:** This option is not provided, so we cannot evaluate it.
* **Option D:** This option is also not provided, so we cannot evaluate it.

**Clinical Pearl / High-Yield Fact**
Remember the pH scale: 0-6.9 is acidic, 7 is neutral, and 7.1-14 is alkaline. A pH of 9 indicates a strong base, which can cause severe chemical burns and other systemic complications.

**Correct Answer: D. 9.**