What solution has a pH of 7?
A.
Aniline
B.
Pyridine
C.
Pure water
D.
Sodium hydroxid
The Correct answer and Explanation is:
The correct answer is C. Pure water.
Explanation:
The pH scale is a measure of the hydrogen ion concentration in a solution, ranging from 0 to 14. A pH of 7 is considered neutral, meaning the concentration of hydrogen ions (H⁺) equals the concentration of hydroxide ions (OH⁻) in the solution.
- Pure Water:
- Pure water has a neutral pH of 7 at 25°C (77°F). This neutrality is due to the fact that water dissociates into hydrogen ions (H⁺) and hydroxide ions (OH⁻) in equal amounts. The equilibrium constant for the dissociation of water is Kw=[H+][OH−]=1.0×10−14K_w = [H^+][OH^-] = 1.0 \times 10^{-14}Kw=[H+][OH−]=1.0×10−14, leading to equal concentrations of [H+][H^+][H+] and [OH−][OH^-][OH−], both being 1.0×10−71.0 \times 10^{-7}1.0×10−7 M. Therefore, the pH is 7, which is neutral.
- Aniline:
- Aniline is a weak base with a pH typically higher than 7. Aniline (C₆H₅NH₂) reacts with water to form aniline hydrochloride and hydroxide ions, increasing the pH of the solution. Thus, aniline does not have a neutral pH.
- Pyridine:
- Pyridine is also a weak base. It reacts with water to form pyridinium ions and hydroxide ions, which results in a basic solution with a pH greater than 7. Therefore, pyridine solutions are not neutral.
- Sodium Hydroxide (NaOH):
- Sodium hydroxide is a strong base that dissociates completely in water to produce hydroxide ions. This leads to a basic solution with a pH well above 7, often much higher depending on the concentration.
In summary, among the options given, pure water is the only substance with a neutral pH of 7. Aniline, pyridine, and sodium hydroxide all result in basic solutions with pH values greater than 7.